Porous membrane and multilayer porous membrane

ABSTRACT

A multilayer porous membrane comprising a porous membrane containing a polyolefin resin as a main component; and a porous layer containing an inorganic filler and a resin binder and laminated on at least one surface of the porous membrane; wherein the porous membrane has an average pore size d=0.035 to 0.060 μm, a tortuosity τ a =1.1 to 1.7, and the number B of pores=100 to 500 pores/μm 2 , which are calculated by a gas-liquid method, and the porous membrane has a membrane thickness L=5 to 22 μm.

TECHNICAL FIELD

The present invention relates to a membrane used for separation,decontamination, and the like of various substances, and a porousmembrane and a multilayer porous membrane suitably used for a separatordisposed between a positive electrode and a negative electrode inbatteries. The present invention further relates to a separator fornonaqueous electrolyte batteries and a nonaqueous electrolyte battery.

BACKGROUND ART

Polyolefin porous membranes are widely used as a separator in batteries,capacitors, and the like because the polyolefin porous membranes showhigh electrical insulation properties and ionic permeability.Particularly, in recent years, lithium ion secondary batteries of highpower density and high capacity density are used as the power supply forportable devices as the functions of the portable devices are increasedand the weight thereof is reduced. The polyolefin porous membrane ismainly used as a separator used for such lithium ion secondarybatteries.

The lithium ion secondary battery has high power density and highcapacity density. Because of an organic solvent used for an electrolyticsolution, however, the electrolytic solution is decomposed by generationof heat accompanied by abnormal situations such as a short circuit andovercharge. This may lead to ignition in the worst case. In order toprevent such situations, some safety functions are incorporated into thelithium ion secondary battery, and one of them is shutdown function ofthe separator. The shutdown function is a function that micro pores ofthe separator are blocked by thermofusion or the like to suppressconduction of ions in the electrolytic solution and to stop progressionof an electrochemical reaction when the battery generates the abnormalheat. Usually, it is supposed that the lower the shutdown temperatureis, the safer the battery is. The proper shutdown temperature ofpolyethylene is one of the reasons that polyethylene is used as acomponent for the separator. A problem of a battery having high energy,however, is that the temperature within the battery continues increasingeven if progression of the electrochemical reaction is stopped byshutdown; as a result, the separator is thermally shrunk and broken,causing a short circuit (short out) between two electrodes.

On the other hand, nonaqueous electrolyte batteries such as lithium ionsecondary batteries have been increasingly applied to electric vehicles,hybrid electric vehicles, and the like, which need charge and dischargeof a large amount of current in a short time. Such applications requirenot only safety but also advanced output properties. Namely, high safetyand advanced output properties need to be satisfied at the same time.

Patent Literature 1 discloses a multilayer porous membrane including aporous membrane composed of a polyolefin resin as a main component and aporous layer laminated on at least one surface of the porous membranewherein the porous layer contains not less than 50% and less than 100%inorganic filler as a mass fraction. The literature describes thetechniques for satisfying high heat resistance for preventingshort-circuit of electrodes and excellent shutdown function at the sametime even if the amount of heat generated is large in abnormal heatgeneration.

Patent Literature 2 discloses a polyolefin microporous membrane having afibril diameter of 40 to 100 nm, a micropore size of 50 to 100 nm, and atortuosity of 1.4 to 1.8. The literature describes the techniques forattaining high ion conductivity and mechanical strength even if apolyolefin microporous membrane and a heat-resistant porous layer areformed into a composite membrane.

LIST OF CITATIONS Patent Literatures Patent Literature 1: JapanesePatent No. 4789274 Patent Literature 2: Japanese Patent Laid-Open No.2011-210574 SUMMARY OF INVENTION Technical Problem

However, the ion conductivities of the conventional multilayer porousmembranes as described in Patent Literature 1 and Patent Literature 2are so insufficient that the requirement for high output properties inapplications of vehicles or the like is not satisfied.

In consideration of such circumstances, an object of the presentinvention is to provide a porous membrane and a multilayer porousmembrane having ion conductivity higher than those of the conventionalmultilayer porous membrane.

Solution to Problem

The present inventors conducted extensive research to solve the problemsabove, and found that a polyolefin porous membrane having a specificpore structure, or a multilayer porous membrane including a polyolefinporous membrane having a specific pore structure and a porous layercontaining an inorganic filler and a resin binder and laminated on thepolyolefin porous membrane has significantly high ion conductivity, andhave achieved the present invention.

Namely, the present invention is as follows.

[1]

A multilayer porous membrane comprising: a porous membrane containing apolyolefin resin as a main component; and a porous layer containing aninorganic filler and a resin binder and laminated on at least onesurface of the porous membrane;

wherein the porous membrane has an average pore size d=0.035 to 0.060μm, a tortuosity τ_(a)=1.1 to 1.7, and the number B of pores=100 to 500pores/μm², which are calculated by a gas-liquid method, and the porousmembrane has a membrane thickness L=5 to 22 μm.

[2]

The multilayer porous membrane according to the above [1], wherein theporous membrane has a porosity ε=50 to 90%.

[3]

The multilayer porous membrane according to the above [1] or [2],wherein the porous membrane comprises a resin composition containingpolypropylene and polyolefin other than the polypropylene.

[4]

The multilayer porous membrane according to the above [3], wherein aproportion of polypropylene based on total polyolefin in the resincomposition is 1 to 35% by mass.

[5]

A separator for nonaqueous electrolyte batteries comprising themultilayer porous membrane according to any one of the above [1] to [4].

[6]

A nonaqueous electrolyte battery comprising the separator for nonaqueouselectrolyte batteries according to the above [5], a positive electrode,a negative electrode, and an electrolytic solution.

[7]

A porous membrane containing a polyolefin resin as a main component,

wherein the porous membrane has a porosity ε=50 to 90% and a shrinkagestress at 85° C. of not more than 2.2 gf.

[8]

A multilayer porous membrane comprising; the porous membrane accordingto the above [7]; and a porous layer containing an inorganic filler anda resin binder and laminated on at least one surface of the porousmembrane.

[9]

The multilayer porous membrane according to the above [8], wherein theporous layer has a thickness of not less than 3 μm and not more than 50μm.

[10]

A separator for nonaqueous electrolyte batteries, comprising the porousmembrane according to the above [7] or the multilayer porous membraneaccording to the above [8] or [9].

[11]

A nonaqueous electrolyte battery, comprising the separator fornonaqueous electrolyte batteries according to the above [10], a positiveelectrode, a negative electrode, and an electrolytic solution.

[12]

A multilayer porous membrane comprising: a porous membrane (A)containing a polyolefin resin as a main component; and a porous layer(B) containing an inorganic filler and a resin binder and laminated onat least one surface of the porous membrane (A);

wherein the porous membrane (A) has a porosity of not less than 50% andnot more than 90%, and the number of pores of not less than 100pores/μm² and not more than 500 pores/μm², and

the resin binder contained in the porous layer (B) is a resin latexbinder having an average particle size of not less than 50 nm and notmore than 500 nm.

[13]

A separator for nonaqueous electrolyte batteries, comprising themultilayer porous membrane according to the above [12].

[14]

A nonaqueous electrolyte battery, comprising the separator fornonaqueous electrolyte batteries according to the above [13], a positiveelectrode, a negative electrode, and an electrolytic solution.

Advantageous Effects of Invention

The present invention can provide a porous membrane and a multilayerporous membrane having high ion conductivity, as well as a separator fornonaqueous electrolyte batteries and a nonaqueous electrolyte batteryincluding the porous membrane or the multilayer porous membrane.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows the results of evaluation on shutdown at a temperatureraising rate of 2° C./min in Example 16 and Comparative Example 24.

FIG. 2 shows the results of evaluation on shutdown at a temperatureraising rate of 18° C./min in Example 16 and Comparative Example 24.

FIG. 3 shows the results of nail penetration evaluation in Example 16.

FIG. 4 shows the results of nail penetration evaluation in ComparativeExample 24.

FIG. 5 is a schematic view showing a laminated body of electrode plates.

DESCRIPTION OF EMBODIMENTS

Now, the present embodiments (hereinafter abbreviated to “Embodiments”)will be described in detail. The present invention will not be limitedto the following embodiments, and various variations and modificationscan be made within the gist of the scope, and implemented.

Embodiment 1

The multilayer porous membrane according to Embodiment 1 is a multilayerporous membrane comprising: a porous membrane containing a polyolefinresin as a main component; and a porous layer containing an inorganicfiller and a resin binder and laminated on at least one surface of theporous membrane; wherein the porous membrane has an average pore sized=0.035 to 0.060 μm and a tortuosity τ_(a)=1.1 to 1.7, and the number Bof pores=100 to 500 pores/μm², which are determined by a gas-liquidmethod, and the porous membrane has a membrane thickness L=5 to 22 μm.

The porous membrane containing a polyolefin resin as a main componentwill be described.

From the viewpoint of improving shutdown performance and the like in thecase where the multilayer porous membrane is used as a separator forbatteries, the porous membrane containing a polyolefin resin as a maincomponent is preferably a porous membrane composed of a polyolefin resincomposition in which the polyolefin resin is not less than 50% and notmore than 100% by mass of the resin components that constitute theporous membrane. The proportion of the polyolefin resin is morepreferably not less than 60% and not more than 100% by mass, and stillmore preferably not less than 70% and not more than 100% by mass.Preferably, the polyolefin resin comprises not less than 50% by mass andnot more than 100% by mass of all components that constitute the porousmembrane.

Examples of the polyolefin resin include, but not limited to,homopolymers, copolymers or multistage polymers of ethylene, propylene,1-butene, 4-methyl-1-pentene, 1-hexene, 1-octene, and the like. Thesepolyolefin resins may be used singly or in combinations of two or more.Specific examples of the polyolefin resin include low densitypolyethylenes, linear low density polyethylenes, midium densitypolyethylenes, high density polyethylenes, ultra high molecular weightpolyethylenes, isotactic polypropylenes, atactic polypropylenes,ethylene-propylene random copolymers, polybutenes, and ethylenepropylene rubbers.

When the multilayer porous membrane is used as a separator forbatteries, a resin composition containing high density polyethylene as amain component is preferably used as the polyolefin resin in particularfrom the viewpoint of satisfying the performance requirements of a lowmelting point and high strength.

More preferably, a resin composition containing polypropylene and apolyolefin resin other than the polypropylene is used from the viewpointof an improvement in the heat resistance of the porous membrane and themultilayer porous membrane.

When polypropylene is contained in the polyolefin resin, thepolypropylene has any steric structure, and may be any of isotacticpolypropylene, syndiotactic polypropylene, and atactic polypropylene.

From the viewpoint of achieving a good balance between heat resistanceand favorable shutdown function, the proportion of polypropylene basedon the total polyolefin in the polyolefin resin composition ispreferably 1 to 35% by mass, more preferably 3 to 20% by mass, and stillmore preferably 4 to 10% by mass. In this case, the polyolefin resincontained other than polypropylene is not limited, and examples thereofinclude homopolymers or copolymers of olefin hydrocarbons such asethylene, 1-butene, 4-methyl-1-pentene, 1-hexene, and 1-octene.Specifically, examples thereof include polyethylenes, polybutenes, andethylene-propylene random copolymers.

In the case where shutdown by blocking pores by thermofusion isrequired, for example, in the case where the multilayer porous membraneis used as a separator for batteries, use of polyethylenes such as lowdensity polyethylenes, linear low density polyethylenes, medium densitypolyethylenes, high density polyethylenes, and ultrahigh molecularweight polyethylenes is preferred as the polyolefin resin other than thepolypropylene. Among these, use of polyethylenes having a density of notless than 0.93 g/cm³, which is measured according to JIS K 7112, is morepreferred from the viewpoint of improving strength.

The viscosity average molecular weight of the polyolefin resin ispreferably not less than 30,000 and not more than 12 million, morepreferably not less than 50,000 and less than 2 million, and still morepreferably not less than 100,000 and less than 1 million. The viscosityaverage molecular weight of not less than 30,000 is preferred becausemelt tension at the time of melt molding tends to be larger to providehigh moldability, and entanglement of the polymers tends to providehigher strength. On the other hand, the viscosity average molecularweight of not more than 12 million is preferred because uniform meltkneading tends to be facilitated to provide higher moldability into thesheet, particularly higher stability in thickness of the sheet. Further,in the case where the multilayer porous membrane according to thepresent embodiment is used as a separator for batteries, the viscosityaverage molecular weight of less than 1 million is preferred becausepores are easily blocked when the temperature is raised and favorableshutdown function tends to be obtained. Instead of using a polyolefinwith a viscosity average molecular weight of less than 1 million alone,for example, a polyolefin mixture of a polyolefin having a viscosityaverage molecular weight of 2 million and a polyolefin having aviscosity average molecular weight of 270,000 wherein the viscosityaverage molecular weight of the polyolefin mixture is less than 1million may be used.

The polyolefin resin composition can contain any additives. Examples ofadditives include polymers other than polyolefin; inorganic filler;antioxidants such as phenol antioxidants, phosphorus antioxidants, andsulfur antioxidants; metallic soaps such as calcium stearate and zincstearate; an ultraviolet absorbing agent; a light stabilizer; anantistatic agent; an anti-fogging agent; and color pigments. The totalamount of these additives to be added is preferably not more than 20parts by mass, more preferably not more than 10 parts by mass, and stillmore preferably not more than 5 parts by mass based on 100 parts by massof the polyolefin resin composition.

The porous membrane according to Embodiment 1 has a multipore structurein which a large number of very fine pores gather to form a finecommunication pore. For such a structure, the porous membrane has veryhigh ion conductivity, good voltage endurance, and high strength. Theporous membrane according to Embodiment 1 is adjusted to have an averagepore size d=0.035 to 0.060 μm, a tortuosity τ_(a)=1.1 to 1.7, and thenumber B of pores=100 to 500 pores/μm², which are determined by thegas-liquid method described later in Examples. When the average poresize d, the tortuosity τ_(a), and the number B of pores are adjustedwithin the ranges above respectively, high ion conductivity and highvoltage endurance can be attained at the same time.

The average pore size d is preferably 0.040 to 0.060 μm, more preferably0.042 to 0.060 μm. The tortuosity τ_(a) is preferably 1.15 to 1.67, morepreferably 1.18 to 1.66, still more preferably 1.20 to 1.65 μm. Thenumber B of pores is preferably 120 to 450 pores/μm², more preferably140 to 400 pores/μm².

The average pore size, the tortuosity, and the number of pores can beadjusted by controlling the composition ratio, the rate of cooling anextruded sheet, the stretching temperature, the stretch ratio, the heatsetting temperature, the stretch ratio during heat setting, and therelaxation rate during heat setting, and combination thereof.

The membrane thickness L of the porous membrane according to Embodiment1 is adjusted to L=5 to 22 μm. A membrane thickness of the porousmembrane of not more than 22 μm reduces membrane resistance, whichadvantageously improves ion conductivity. The effect is remarkable whenthe porous layer is laminated to form a multilayer porous membrane. Amembrane thickness of the porous membrane of not less than 5 μm improvesvoltage endurance. The membrane thickness of the porous membrane ispreferably 6 to 21 μm, more preferably, 7 to 20 μm. The membranethickness of the porous membrane can be adjusted by controlling thestretch ratio at a stretching step, or the like.

The porous membrane according to Embodiment 1 preferably has a porosityε=50 to 90%. At a porosity of the porous membrane of not less than 50%,ion conductivity tends to improve, as membrane resistance tends toreduce. At a porosity of the porous membrane of not more than 90%,voltage endurance tends to improve. The porosity of the porous membraneis more preferably 50 to 80%, still more preferably 52 to 75%,particularly preferably 55 to 70%. The porosity of the porous membranecan be adjusted by controlling the mixing ratio of the polyolefin resinto a plasticizer, the stretching temperature, the stretch ratio, theheat setting temperature, the stretch ratio during heat setting, and therelaxation rate during heat setting, and combination thereof.

The puncture strength of the porous membrane according to Embodiment 1is preferably 400 to 2000 gf in terms of a membrane thickness of 25 μm.The puncture strength of the porous membrane is more preferably 420 to1800 gf, still more preferably 450 to 1500 gf, particularly preferably500 to 1200 gf. The puncture strength of the porous membrane can beadjusted by controlling the kind of the polyolefin resin and thecomposition ratio thereof, the rate of cooling the extruded sheet, thestretching temperature, the stretch ratio, the heat setting temperature,the stretch ratio during heat setting, and the relaxation rate duringheat setting, and combination thereof.

The process for producing the porous membrane according to Embodiment 1is not particularly limited, but any known production process can beapplied. Examples thereof include (1) a process for porosifying bymelting and kneading a polyolefin resin composition and a plasticizer tomold the kneaded product into a sheet form, stretching the sheet whennecessary, and extracting the plasticizer; (2) a process for porosifyingby melting and kneading a polyolefin resin composition, extruding thekneaded product at a high draw ratio, and peeling off a polyolefincrystal interface by heat treatment and stretching; (3) a process forporosifying by melting and kneading a polyolefin resin composition andan inorganic filler, molding the kneaded product into a sheet form, andstretching the molded product to peel off the interface between thepolyolefin and the inorganic filler; and (4) a process for porosifyingby dissolving a polyolefin resin composition, and removing a solventsimultaneously with immersion in a poor solvent to the polyolefin tosolidify the polyolefin.

Hereinafter, a process for melting and kneading a polyolefin resincomposition and a plasticizer, molding the kneaded product into a sheetform, and extracting the plasticizer will be described as an example ofthe process for producing a porous membrane.

First, the polyolefin resin composition and the plasticizer are moltenand kneaded. Examples of the melting kneading process include a processcomprising: feeding a polyolefin resin and other additives whennecessary to a resin kneading apparatus such as an extruder, a kneader,a Labo Plast mill, a mixing roll, or a Banbury mixer; and introducingand kneading a plasticizer at an arbitrary proportion while heat meltingthe resin component. Prior to feeding to the resin kneading apparatus,the polyolefin resin, the other additives, and the plasticizer arepreferably kneaded in advance at a predetermined proportion using aHenschel mixer or the like. More preferably, the plasticizer ispartially fed in the advance kneading, and the rest of the plasticizeris kneaded while being side fed in the resin kneading apparatus. Use ofsuch a kneading method can enhance the dispersibility of theplasticizer. Thereby, a melt kneaded product of the resin compositionand the plasticizer tends to be able to be stretched into a sheet-likemolded product in a subsequent step at a high stretch ratio withoutbeing broken.

Any plasticizer can be used without particular limitation, and anon-volatile solvent that can form a uniform solution even at atemperature equal to or higher than a melting point of the polyolefin ispreferably used. Specific examples of such a nonvolatile solventinclude: hydrocarbons such as liquid paraffin and paraffin wax; esterssuch as dioctyl phthalate and dibutyl phthalate; and higher alcoholssuch as oleyl alcohol and stearyl alcohol.

Among these, preferred is liquid paraffin because it has highcompatibility with polyethylene and polypropylene and causes no orlittle peeling at the interface between the resin and the plasticizer bystretching the molten and kneaded product, and thereby uniformstretching tends to be performed.

The proportion of the plasticizer to the polyolefin resin composition isnot particularly limited, and the plasticizer can be added at anyproportion in which the polyolefin resin composition and the plasticizercan be uniformly molten and kneaded to be molded into a sheet form. Forexample, the mass fraction of the plasticizer based on the compositioncomposed of the polyolefin resin composition and the plasticizer ispreferably 30 to 80% by mass, and more preferably 40 to 70% by mass. Ata mass fraction of the plasticizer of not more than 80% by mass,insufficient melt tension at the time of melt molding tends not to becaused, leading to improved moldability. On the other hand, at a massfraction of the plasticizer of not less than 30% by mass, polyolefinchains are not broken by stretching the mixture of the polyolefin resincomposition and the plasticizer even at a high stretch ratio, easilyleading to a uniform and micro pore structure. Moreover, strength tendsto be increased.

Next, the molten and kneaded product is molded into a sheet form.Examples of a process for producing a sheet-like molded product includea process comprising extruding a molten and kneaded product into a sheetform through a T die or the like, contacting the extruded product with aheat conductive body to cool the extruded product to a temperaturesufficiently lower than the crystallization temperature of the resincomponent and solidify the extruded product. Examples of a heatconductor used for cooling and solidification include metals, water,air, or the plasticizer itself. A metallic roll is preferably usedbecause such a roll has high heat conductivity. More preferably, theextruded kneaded product is sandwiched between the metallic rolls whenbrought into contact with the metallic rolls because the heatconductivity is further increased, and the sheet tends to be oriented tohave increased membrane strength and improved surface smoothness of thesheet. When the melt kneaded product is extruded into a sheet from a Tdie, the die lip clearance is preferably not less than 400 μm and notmore than 3000 μm, more preferably not less than 500 μm and not morethan 2500 μm. At a die lip clearance of not less than 400 μm, die drooland the like can be reduced to decrease adverse influences on thequality of the membrane such as streaks and defects, and reduce the riskof breakage of the membrane in the subsequent stretching step. On theother hand, at a die lip clearance of not more than 3000 μm, the coolingrate is higher to prevent unevenness in cooling and maintain thestability in thickness of the sheet.

Next, preferably, the thus-obtained sheet-like molded product isstretched. As the stretching treatment, either uniaxial stretching orbiaxial stretching can be used suitably. From the viewpoint of improvingstrength of the porous membrane obtained, biaxial stretching ispreferred. By stretching of the sheet-like molded product in two axialdirections at a high stretch ratio, molecules are oriented in the planardirection, and thereby the porous membrane eventually obtained isdifficult to tear and obtains high puncture strength. Examples of thestretching can include processes such as simultaneous biaxialstretching, successive biaxial stretching, multi stage stretching, andstretching several times. From the viewpoint of improvement in puncturestrength, uniformity of stretching, and shutdown properties, thesimultaneous biaxial stretching is preferred.

Here, the simultaneous biaxial stretching refers to a stretching processthat simultaneously performs stretching in the MD (machine direction ofthe microporous membrane) and stretching in the TD (transverse directionintersecting the MD of the microporous membrane at an angle of 90°). Thestretch ratios in the directions may be different. The successivebiaxial stretching refers to a stretching process that performsstretching in the MD or that in the TD independently. When stretching isperformed in one of the MD and the TD, the microporous membrane isnon-restrained or fixed at a fixed length in the other directionthereof.

The stretch ratio is preferably in the range of not less than 20 timesand not more than 100 times, and more preferably in the range of notless than 25 times and not more than 50 times in terms of an area ratio.The stretch ratio in each axial direction is preferably in the range ofnot less than 4 times and not more than 10 times in the MD and not lessthan 4 times and not more than 10 times in the TD, and more preferablyin the range of not less than 5 times and not more than 8 times in theMD and not less than 5 times and not more than 8 times in the TD. At atotal area ratio of not less than 20 times, sufficient strength tends tobe given to the porous membrane obtained. On the other hand, at a totalarea ratio of not more than 100 times, breakage of the membrane at thestretching step tends to be prevented, and high productivity tends to beobtained.

The sheet-like molded product may be rolled. Rolling can be performed bya pressing that uses a double belt press machine, for example. Rollingcan particularly increase orientation of a layer portion. The rollingarea ratio is preferably more than 1 time and not more than 3 times, andmore preferably more than 1 time and not more than 2 times. At a rollingratio greater than 1 time, plane orientation tends to be increased toincrease membrane strength of the porous membrane eventually obtained.On the other hand, when a rolling ratio is not more than 3 times, aporous structure having a small difference between the orientation ofthe layer portion and that of an inner central portion and uniformity inthe thickness direction of the membrane tends to be formed.

Next, the plasticizer is removed from the sheet-like molded product toobtain a porous membrane. Examples of the process for removing aplasticizer include a process comprising immersing a sheet-like moldedproduct in an extraction solvent to extract a plasticizer, and dryingsufficiently. The process for extracting a plasticizer may be a batchprocess or a continuous process. In order to suppress shrinkage of theporous membrane, edges of the sheet-like molded product are preferablyrestrained during the series of steps of immersing and drying. Theamount of the plasticizer remaining in the porous membrane is preferablyless than 1% by mass based on the total mass of the porous membrane.

The solvent used for extraction of the plasticizer is preferably a poorsolvent to the polyolefin resin and a good solvent to the plasticizer,and has a boiling point lower than the melting point of the polyolefinresin. Examples of such an extraction solvent include: hydrocarbons suchas n-hexane and cyclohexane; halogenated hydrocarbons such as methylenechloride and 1,1,1-trichloroethane; non-chlorine halogenated solventssuch as hydrofluoroethers and hydrofluorocarbons; alcohols such asethanol and isopropanol; ethers such as diethyl ether andtetrahydrofuran; and ketones such as acetone and methyl ethyl ketone.These extraction solvents may be recovered by operation such asdistillation to be reused.

In order to suppress shrinkage of the porous membrane, a heat treatmentsuch as heat setting and thermal relaxation can also be performed afterthe stretching step or formation of the porous membrane. Alternatively,the porous membrane may be subjected to post-treatments such ashydrophilization treatment by a surface active agent, a crosslinkingprocess by ionizing radiation, or the like.

The porous membrane is preferably treated with heat setting from theviewpoint of suppression of shrinkage. Examples of the heat settingmethod include relaxation operation performed in an atmosphere at apredetermined temperature and a predetermined relaxation rate, which canuses a tenter or a roll stretching machine. The relaxation operationrefers to an operation of contraction of the membrane in MD and/or TD.The relaxation rate refers to a value obtained by dividing the size ofthe membrane in MD after the relaxation operation by the size of themembrane in MD before the operation, a value obtained by dividing thesize of the membrane in TD after the relaxation operation by the size ofthe membrane in TD before the operation, or a value obtained bymultiplying the relaxation rate in MD by the relaxation rate in TD whenthe relaxation operation is performed in both MD and TD. The relaxationrate is preferably not more than 1.0, more preferably not more than0.97, still more preferably not more than 0.95.

The relaxation operation may be performed in both MD and TD, or may beperformed in one of MD and TD. Before the relaxation operation, themembrane is stretched 1.8 times or more, more preferably 2.0 times ormore in MD and/or TD to readily attain a porous membrane having highstrength and high porosity. The stretching and relaxation operationsafter the extraction of the plasticizer are performed preferably in TD.The temperature in the relaxation operation and that in the stretchingstep before the relaxation operation are preferably lower than themelting point (Tm) of the polyolefin resin, more preferably in the rangeof Tm−5° C. to Tm−25° C., still more preferably in the range of Tm−7° C.to Tm−23° C., particularly preferably in the range of Tm−8° C. to Tm−21°C. The temperature in the relaxation operation and that in thestretching step before the relaxation operation within the ranges abovereadily attain a porous membrane having a small pore size, a lowtortuosity, a larger number of pores, and high porosity.

Next, a porous layer containing an inorganic filler and a resin binderwill be described.

Any inorganic filler can be used in the porous layer without particularlimitation. A preferable inorganic filler has a melting point of notless than 200° C., high electrical insulation properties, andelectrochemical stability in the conditions of lithium ion secondarybatteries used.

Examples of the inorganic filler include: oxide ceramics such asalumina, silica, titania, zirconia, magnesia, ceria, yttria, zinc oxide,and iron oxide; nitride ceramics such as silicon nitride, titaniumnitride, and boron nitride; ceramics such as silicon carbide, calciumcarbonate, magnesium sulfate, aluminum sulfate, aluminum hydroxide,aluminum hydroxide oxide, potassium titanate, talc, kaolinite, dickite,nacrite, halloysite, pyrophyllite, montmorillonite, sericite, mica,amesite, bentonite, asbestos, zeolite, calcium silicate, magnesiumsilicate, diatomaceous earth, and quartz sand; and glass fibers. Thesemay be used singly or in combinations of two or more.

Among these, aluminum oxide compounds such as alumina and aluminumhydroxide oxide, and aluminum silicate compounds without ionexchangeability such as kaolinite, dickite, nacrite, halloysite, andpyrophyllite are preferable from the viewpoint of an improvement inelectrochemical stability and the heat resistance properties of themultilayer porous membrane. For the aluminum oxide compounds, aluminumhydroxide oxide is particularly preferable. For the aluminum silicatecompounds having no ion exchangeability, kaolin composed of mainlykaolin mineral is more preferable because kaolin is inexpensive andreadily available. Kaolin includes wet kaolin and calcined kaolinprepared by calcining wet kaolin. Calcined kaolin is particularlypreferable from the viewpoint of electrochemical stability becausecrystal water is discharged and impurities are removed duringcalcination.

The inorganic filler preferably has an average particle size of morethan 0.1 μm and not more than 4.0 μm, more preferably more than 0.2 μmand not more than 3.5 μm, still more preferably more than 0.4 μm and notmore than 3.0 μm. Adjustment of the average particle size of theinorganic filler within the range above is preferable from the viewpointof suppression of thermal shrinkage at high temperatures even if theporous layer is thin (for example, not more than 7 μm).

In the inorganic fillers, the proportion of the particles having aparticle size of more than 0.2 μm and not more than 1.4 μm based on theentire inorganic fillers is preferably not less than 2% by volume, morepreferably not less than 3% by volume, still more preferably not lessthan 5% by volume, the upper limit is preferably not more than 90% byvolume, and more preferably not more than 80% by volume.

In the inorganic fillers, the proportion of particles having a particlesize of more than 0.2 μm and not more than 1.0 μm based on the entireinorganic fillers is preferably not less than 1% by volume, morepreferably not less than 2% by volume, the upper limit is preferably notmore than 80% by volume, and more preferably not more than 70% byvolume.

In the inorganic fillers, the proportion of particles having a particlesize of more than 0.5 μm and not more than 2.0 μm based on the entireinorganic fillers is preferably not less than 8% by volume, morepreferably not less than 10% by volume, the upper limit is preferablynot more than 60% by volume, and more preferably not more than 50% byvolume.

Further, in the inorganic fillers, the proportion of particles having aparticle size of more than 0.6 μm and not more than 1.4 μm based on theentire inorganic fillers is preferably not less than 1% by volume, morepreferably not less than 3% by volume, the upper limit is preferably notmore than 40% by volume, and more preferably not more than 30% byvolume.

Adjustment of the particle size distribution of the inorganic fillerwithin the range above is preferable from the viewpoint of suppressionof thermal shrinkage at high temperatures even if the porous layer isthin (for example, not more than 7 μm). Examples of the method ofadjusting the proportion of the particle size of the inorganic fillercan include a method of crushing an inorganic filler with a ball mill, abead mill, a jet mill, or the like to reduce the particle size.

Examples of the shape of the inorganic filler include plate shapes,flake shapes, needle shapes, column shapes, spherical shapes, polyhedricshapes, and bulk shapes. Inorganic fillers having the shapes above maybe used in combination. The inorganic filler has any shape withoutparticular limitation as long as the inorganic filler can suppressthermal shrinkage at 150° C. of the multilayer porous membrane describedlater to be not more than 10%. Polyhedric shapes, column shapes, andspindle shapes having several planes are preferable from the viewpointof an improvement in permeability.

From the viewpoint of binding properties of the inorganic fillers andpermeability and heat resistance of the multilayer porous membrane, theproportion of the inorganic fillers in the porous layer can be properlydetermined, and is preferably not less than 50% and less than 100% bymass, more preferably not less than 70% and not more than 99.99% bymass, still more preferably not less than 80% and not more than 99.9% bymass, and particularly more preferably not less than 90% and not morethan 99% by mass.

While the kind of resin binder is not particularly limited, use of aresin binder insoluble to the electrolytic solution of the lithium ionsecondary battery and electrochemically stable in the use conditions ofthe lithium ion secondary battery is preferred in the case where themultilayer porous membrane according to the present embodiment is usedas a separator for lithium ion secondary batteries.

Specific examples of a resin binder include: polyolefins such aspolyethylene and polypropylene; fluorine containing resins such aspolyvinylidene fluoride and polytetrafluoroethylene; fluorine containingrubbers such as vinylidenefluoride-hexafluoropropylene-tetrafluoroethylene copolymers andethylene-tetrafluoroethylene copolymers; rubbers such asstyrene-butadiene copolymers and hydrides thereof,acrylonitrile-butadiene copolymers and hydrides thereof,acrylonitrile-butadiene-styrene copolymers and hydrides thereof,methacrylic acid ester-acrylic acid ester copolymers, styrene-acrylicacid ester copolymers, acrylonitrile-acrylic acid ester copolymers,ethylene propylene rubber, polyvinyl alcohol, and polyvinyl acetate;cellulose derivatives such as ethyl cellulose, methyl cellulose,hydroxyethyl cellulose, and carboxymethyl cellulose; and resins having amelting point and/or a glass transition temperature of not less than180° C. such as polyphenylene ether, polysulfone, polyethersulfone,polyphenylene sulfide, polyetherimide, polyamidoimide, polyamide, andpolyester.

In the case where polyvinyl alcohol is used as a resin binder, thedegree of saponification is preferably not less than 85% and not morethan 100%. A degree of saponification of not less than 85% is preferredbecause an improved short-circuit temperature (short out temperature)and higher safe performance are tend to be obtained when the multilayerporous membrane is used as a separator for batteries. The degree ofsaponification is more preferably not less than 90% and not more than100%, still more preferably not less than 95% and not more than 100%,and particularly more preferably not less than 99% and not more than100%. The degree of polymerization of polyvinyl alcohol is preferablynot less than 200 and not more than 5000, more preferably not less than300 and not more than 4000, still more preferably not less than 500 andnot more than 3500. Preferably, the degree of polymerization is not lessthan 200, because a small amount of polyvinyl alcohol can firmly bondthe inorganic filler such as calcined kaolin to the porous membrane, andtends to be able to suppress an increase in air permeability of themultilayer porous membrane caused by formation of the porous layer whilethe mechanical strength of the porous layer is maintained. Also, thedegree of polymerization of not more than 5000 is preferred becausegelation and the like in preparation of a coating solution tend to beprevented.

For the resin binder, a resin latex binder is preferred. When the resinlatex binder is used and a porous layer containing an inorganic fillerand a binder is laminated on at least one surface of the polyolefinporous membrane, ionic permeability is not easily reduced and highoutput properties are easily obtained. Also when the temperature risesrapidly in abnormal heat generation, smooth shutdown properties aredemonstrated to readily attain high safety. On the other hand, when partor all of the resin binder is dissolved in a solvent to prepare asolution, the solution is laminated on at least one surface of thepolyolefin porous membrane, and the resin binder is bonded to the porousmembrane by, e.g., removing the solvent by immersion of the membrane ina poor solvent or drying of the membrane, high output properties aredifficult to obtain and smooth shutdown properties are difficult todemonstrate, which tends to lead to poor safety.

A preferable resin latex binder is those prepared by emulsionpolymerization of: an aliphatic conjugated diene monomer; an unsaturatedcarboxylic acid monomer; and other monomers copolymerizable with theabove monomers, from the viewpoint of an improvement in electrochemicalstability and bonding properties. Any emulsion polymerization method canbe used without particular limitation, and a conventionally known methodcan be used. Any method of adding a monomer and other components can beused without particular limitation, and any of a batch addition method,a divided addition method, and a continuous addition method can be used.Any of one-stage polymerization, two-stage polymerization, multistagepolymerization, and the like can be used.

Examples of the aliphatic conjugated diene monomer include, but notlimited to, 1,3-butadiene, 2-methyl-1,3-butadiene,2,3-dimethyl-1,3-butadiene, 2-chloro-1,3-butadiene, substituted linearchain conjugated pentadienes, and substituted and side chain conjugatedhexadienes. These may be used singly or in combinations of two or more.Among these, 1,3-butadiene is particularly preferable.

Examples of the unsaturated carboxylic acid monomer include, but notlimited to, mono- or dicarboxylic acids (anhydrides) such as acrylicacid, methacrylic acid, crotonic acid, maleic acid, fumaric acid, anditaconic acid. These may be used singly or in combinations of two ormore. Among these, acrylic acid and methacrylic acid are particularlypreferable.

Examples of other monomers copolymerizable with the monomers aboveinclude, but not limited to, aromatic vinyl monomers, vinyl cyanidemonomers, unsaturated carboxylic acid alkyl ester monomers, unsaturatedmonomers having hydroxy alkyl groups, and unsaturated carboxylic acidamide monomers. These may be used singly or in combinations of two ormore. Among these, unsaturated carboxylic acid alkyl ester monomers areparticularly preferable. Examples of the unsaturated carboxylic acidalkyl ester monomer include methyl acrylate, methyl methacrylate, ethylacrylate, ethyl methacrylate, butyl acrylate, glycidyl methacrylate,dimethyl fumarate, diethyl fumarate, dimethyl maleate, diethyl maleate,dimethyl itaconate, monomethyl fumarate, monoethyl fumarate, and2-ethylhexyl acrylate. These may be used singly or in combinations oftwo or more. Among these, methyl methacrylate is particularlypreferable.

In addition to these monomers, monomer components other than themonomers above can also be used to improve various quality and physicalproperties.

In Embodiment 1, the resin binder preferably has an average particlesize of 50 to 500 nm, more preferably 60 to 460 nm, and still morepreferably 80 to 250 nm. At an average particle size of the resin binderof not less than 50 nm, ionic permeability is not easily reduced andhigh output properties are easily obtained when the porous layercontaining an inorganic filler and a binder is laminated on at least onesurface of the polyolefin porous membrane. Also when the temperaturerises rapidly in abnormal heat generation, smooth shutdown propertiesare demonstrated to readily attain high safety. At an average particlesize of the resin binder of not more than 500 nm, high bondingproperties are demonstrated, and the multilayer porous membrane tends toattain good thermal shrinkage and high safety.

The average particle size of the resin binder can be controlled byadjusting the polymerization time, the polymerization temperature, thecomposition ratio of raw materials, the order of feeding the rawmaterials, the pH, and the like.

The layer thickness of the porous layer is preferably not less than 1 μmfrom the viewpoint of an improvement in heat resistance and electricalinsulation properties, and preferably not more than 50 μm from theviewpoint of higher capacity of the battery and an improvement inpermeability. The layer thickness of the porous layer is more preferablynot less than 1.5 μm and not more than 20 μm, more preferably not lessthan 2 μm and not more than 10 μm, still more preferably not less than 3μm and not more than 10 μm, particularly preferably not less than 3 μmand not more than 7 μm.

The layer density of the porous layer is preferably 0.5 to 2.0 g/cm³,and more preferably 0.7 to 1.5 cm³. At a layer density of the porouslayer of not less than 0.5 g/cm³, the thermal shrinkage rate at hightemperatures tends to be improved. At a layer density of not more than2.0 g/cm³, air permeability tends to be reduced.

Examples of a process for forming a porous layer can include a processof applying a coating solution containing inorganic fillers and a resinbinder on at least one surface of a porous membrane containing apolyolefin resin as a main component to form a porous layer.

As a solvent for the coating solution, those that can disperse theinorganic fillers and the resin binder uniformly and stably arepreferred. Examples thereof include N-methylpyrrolidone,N,N-dimethylformamide, N,N-dimethylacetamide, water, ethanol, toluene,hot xylene, methylene chloride, and hexane.

In order to stabilize distribution and to improve applicability, variousadditives such as a dispersing agent such as a surface active agent; athickener; a wetting agent; an antifoaming agent; a pH adjusting agentcontaining an acid or an alkali may be added to the coating solution.Among these additives, the additives that can be removed at the time ofremoving the solvent are preferred. The porous layer, however, may haveany remaining additive that is electrochemically stable in the useconditions of the lithium ion secondary battery, does not obstruct acell reaction, and is stable to approximately 200° C.

A method for dispersing the inorganic fillers and the resin binder in asolvent for a coating solution is not particularly limited as long asthe method can realize dispersion properties of the coating solutionnecessary for an application step. Examples thereof include mechanicalstirring by a ball mill, a bead mill, a planet ball mill, a vibrationball mill, a sand mill, a colloid mill, an attritor, a roll mill,high-speed impeller distribution, a disperser, a homogenizer, ahigh-speed impact mill, ultrasonic dispersion, and an impeller.

A method for applying a coating solution to a porous membrane is notparticularly limited as long as it can provide a necessary layerthickness and area to be applied. Examples thereof include a gravurecoater method, a small diameter gravure coater method, a reverse rollcoater method, a transfer roll coater method, a kiss coater method, adip coater method, a knife coater method, an air doctor coater method, ablade coater method, a rod coater method, a squeeze coater method, acast coater method, a die coater method, screen printing, and spraycoating.

Further, performing a surface treatment on the surface of the porousmembrane prior to applying a coating solution is preferred because thecoating solution is easily applied and adhesiveness between theinorganic filler containing porous layer and the surface of the porousmembrane after application is improved. A process for the surfacetreatment is not particularly limited as long as it does not impair theporous structure of the porous membrane remarkably. Examples thereofinclude corona discharge treatment, mechanical surface roughening, asolvent treatment, an acid treatment, and ultraviolet rays oxidation.

A process for removing a solvent from a coated membrane afterapplication is not particularly limited as long as it does not have anadverse influence on the porous membrane. Examples thereof include aprocess for drying a porous membrane at a temperature not more than themelting point thereof and a process for performing reduced pressuredrying at a low temperature, while fixing a porous membrane. Preferably,the drying temperature, a tension in winding, and the like are properlyadjusted from the viewpoint of controlling shrinkage stress applied inMD of the porous membrane and in MD of the multilayer porous membrane.

Embodiment 2

The porous membrane according to Embodiment 2 is a porous membranecontaining a polyolefin resin as a main component,

wherein the porous membrane has a porosity ε=50 to 90%, and the porousmembrane has a shrinkage stress at 85° C. of not more than 2.2 gf.

The same polyolefin resin and additives as those described in Embodiment1 can be contained in the porous membrane according to Embodiment 2.

The shrinkage stress at 85° C. of the porous membrane according toEmbodiment 2 is adjusted to not more than 2.2 gf.

Entry of moisture into the nonaqueous electrolyte battery can cause areduction of the cycle life and the capacity of the battery. Thus, adrying step is often performed before the electrolytic solution ispoured into the battery. The drying step often involves heating.Formation of the porous layer containing an inorganic filler and a resinbinder provides a great effect of suppressing thermal shrinkage todecrease shrinkage of the separator. However, probably in the process inwhich the porous membrane is once heated in the drying step and returnedto normal temperature, the polyolefin resin flows in a complex manner tochange the pore structure. Probably, this phenomenon becomes remarkableparticularly when the porosity is high. On the basis of the knowledge,the present inventors found that a polyolefin porous membrane having ashrinkage stress at a predetermined temperature adjusted in not morethan a specific range can attain high output properties and uniformityat the same time. The present inventors also found that the porous layercontaining an inorganic filler and a resin binder is laminated on theporous membrane to attain higher output properties and uniformity at thesame time.

The shrinkage stress at 85° C. of the porous membrane is preferably notmore than 2 gf, and more preferably not more than 1.8 gf. The shrinkagestress can be determined by the method (TMA) described later inExamples. The shrinkage stress refers to the shrinkage stress in both MD(machine direction) and TD (transverse direction crossing the machinedirection at 90°), and the shrinkage stress in MD and that in TD areadjusted to not more than 2.2 gf. At a shrinkage stress at 85° C. of notmore than 2.2 gf, the polyolefin pore structure barely changes duringdrying the battery, leading to advantages in that ion conductivity andthe uniformity of ion conductivity are difficult to impair.

An effective method of adjusting the shrinkage stress of the porousmembrane to not more than 2.2 gf is thermal relaxation. For example, toprepare a porous membrane having a porosity of not less than 50% and ashrinkage stress at 85° C. of not more than 2.2 gf, preferably, a porousmembrane having a high porosity of not less than 51% is prepared, and isthen subjected to thermal relaxation at a relaxation rate of less than1.0. The control of shrinkage stress by thermal relaxation can beconducted by adjusting the relaxation rate, the heat treatmenttemperature, strength, the polyolefin resin composition, and the like.

The porosity ε of the porous membrane according to Embodiment 2 isadjusted to 50 to 90%. At a porosity of the porous membrane of not lessthan 50%, membrane resistance tends to be reduced, and thus therewithion conductivity tends to be improved. At a porosity of the porousmembrane of not more than 90%, puncture strength tends to be improved.The porous membrane preferably has a porosity of 50 to 80%, morepreferably 52 to 75%, and still more preferably 55 to 70%. The porosityof the porous membrane can be adjusted by controlling the mixing ratioof the polyolefin resin to the plasticizer, the stretching temperature,the stretch ratio, the heat setting temperature, the stretch ratioduring heat setting, and the relaxation rate during heat setting, andcombination thereof.

The description of the physical properties and the production method ofthe porous membrane according to Embodiment 2 other than above are thesame as those in Embodiment 1. The description of the porous layercontaining an inorganic filler and a resin binder is the same as that inEmbodiment 1.

Embodiment 3

The multilayer porous membrane according to Embodiment 3 is a multilayerporous membrane including a porous membrane (A) containing a polyolefinresin as a main component, and a porous layer (B) containing aninorganic filler and a resin binder and laminated on at least onesurface of the porous membrane,

wherein a porosity of the porous membrane (A) is not less than 50% andnot more than 90%, and the number of pores is not less than 100pores/μm² and not more than 500 pores/μm², and

the resin binder in the porous layer (B) is a resin latex binder havingan average particle size of not less than 50 nm and not more than 500nm.

The same polyolefin resin and additives as those in Embodiment 1 can becontained in the porous membrane according to Embodiment 3.

In Embodiment 3, the porosity of the porous membrane (A) containing apolyolefin resin as a main component is adjusted to not less than 50%and not more than 90%. A porosity of not less than 50% attains highionic permeability, leading to high output properties when themultilayer porous membrane is used as a separator for nonaqueouselectrolyte batteries. A porosity of not more than 90% reduces the riskof self-discharge and improves reliability when the multilayer porousmembrane is used as a separator for nonaqueous electrolyte batteries.The porosity of the porous membrane (A) is preferably not less than 50%and not more than 80%, more preferably not less than 52% and not morethan 75%, and still more preferably not less than 55% and not more than70%.

The porosity can be adjusted by controlling the mixing ratio of thepolyolefin resin to the plasticizer, the draw ratio of the extrudedsheet, the stretching temperature, the stretch ratio, the heat settingtemperature, the stretch ratio during heat setting, and the relaxationrate during heat setting, and combining these.

The number of pores per unit area of the porous membrane (A) is adjustedto not less than 100 pores/μm² and not more than 500 pores/μm². If thenumber of pores is not less than 100 pores/μm² and not more than 500pores/μm², high output properties are attained because a reduction inpermeability caused by penetration, clogging or the like of theinorganic filler and the binder is small when a porous layer (B)containing an inorganic filler and a binder is formed on the surface ofthe porous membrane (A). Also, high safety is attained, becauseexcellent shutdown properties are demonstrated when the temperatureraising rate is significantly high due to abnormal heat generation. Thelower limit of the number of pores in the porous membrane (A) ispreferably not less than 120 pores/μm², and more preferably not lessthan 130 pores/μm², and the upper limit is preferably not more than 460pores/μm², and more preferably not more than 400 pores/μm².

The number of pores can be adjusted by controlling the mixing ratio ofthe polyolefin resin to the plasticizer, the rate of cooling theextruded sheet, the degree of rolling the extruded sheet, the stretchingtemperature, the stretch ratio, the heat setting temperature, thestretch ratio during heat setting, and the relaxation rate during heatsetting, and combining these. In particular, the number of pores issignificantly influenced by the stretching temperature, the stretchratio, and the heat setting temperature.

The membrane thickness of the porous membrane (A) is preferably in therange of not less than 2 μm and not more than 40 μm, and more preferablynot less than 5 μm and not more than 35 μm. A membrane thickness of notless than 2 μm tends to attain sufficient mechanical strength. Amembrane thickness of not more than 40 μm advantageously tends to attainhigher capacity of the battery because the volume occupied by theseparator is reduced.

The air permeability of the porous membrane (A) is preferably not lessthan 10 seconds and not more than 500 seconds, and more preferably notless than 20 seconds and not more than 400 seconds. An air permeabilityof not less than 10 seconds tends to reduce self-discharge when themultilayer porous membrane is used as a separator for battery. An airpermeability of not more than 500 seconds tends to attain good chargeand discharge properties.

The pore size of the porous membrane (A) is preferably 0.01 to 3 μm,more preferably 0.02 to 1 μm, and still more preferably 0.035 to 0.060μm. A pore size of not less than 0.01 μm tends to attain high outputproperties because a reduction in permeability caused by penetration,clogging or the like of the inorganic filler and the binder is smallwhen a porous layer (B) containing an inorganic filler and a binder isformed on the surface of the porous membrane (A). A pore size of notmore than 5 μm tends to reduce self-discharge when the multilayer porousmembrane is used as a separator for batteries.

The pore size can be adjusted by controlling the mixing ratio of thepolyolefin resin to the plasticizer, the rate of cooling the extrudedsheet, the stretching temperature, the stretch ratio, the heat settingtemperature, the stretch ratio during heat setting, and the relaxationrate during heat setting, and combining these. In particular, the poresize is significantly influenced by the mixing ratio of the polyolefinresin to the plasticizer, the stretching temperature, the stretch ratio,and the heat setting temperature.

Next, the porous layer (B) containing an inorganic filler and a resinbinder will be described.

The same inorganic filler and resin binder as those described inEmbodiment 1 can be contained in the porous layer (B) according toEmbodiment 3.

In Embodiment 3, a resin latex binder is used as a resin binder. Asdescribed herein, the term “resin latex” refers to a resin dispersed ina medium. The resin latex binder barely reduces ionic permeability andreadily attains high output properties when the porous layer (B)containing an inorganic filler and a resin binder is laminated on atleast one surface of the polyolefin porous membrane. Also when thetemperature rises rapidly in abnormal heat generation, smooth shutdownproperties are demonstrated to readily attain high safety.

In Embodiment 3, the average particle size of the resin latex binder isadjusted to 50 to 500 nm. The average particle size of the resin latexbinder is preferably 60 to 460 nm, and more preferably 80 to 220 nm. Anaverage particle size of not less than 50 nm barely reduces ionicpermeability and readily attains high output properties when the porouslayer (B) containing an inorganic filler and a resin binder is laminatedon at least one surface of the polyolefin porous membrane. Also when thetemperature rises rapidly in abnormal heat generation, smooth shutdownproperties are demonstrated to readily attain high safety. An averageparticle size of not more than 500 nm attains high bonding properties,which tends to lead to good thermal shrinkage and high safety when theporous layer is used in the multilayer porous membrane.

The average particle size of the resin latex binder can be controlled byadjusting the polymerization time, the polymerization temperature, thecomposition ratio of raw materials, the order of feeding raw materials,the pH, and the like.

The layer thickness of the porous layer (B) is preferably not less than1 μm from the viewpoint of an improvement in heat resistance andelectrical insulation properties, and preferably not more than 50 μmfrom the viewpoint of obtaining higher capacity of the battery and animprovement in permeability. The layer thickness of the porous layer ismore preferably not less than 1.5 μm and not more than 20 μm, morepreferably not less than 2 μm and not more than 10 μm, still morepreferably not less than 3 μm and not more than 10 μm, and particularlypreferably not less than 3 μm and not more than 7 μm.

The description of the method for forming the porous layer (B) issimilar to that in Embodiment 1.

Next, cases where the porous membranes or the multilayer porousmembranes according to Embodiments 1 to 3 are used as a separator forbatteries will be described.

The porous membranes or the multilayer porous membranes according toEmbodiments have high heat resistance and shutdown function, and aresuitable for a separator for batteries that separates a positiveelectrode from a negative electrode in the battery.

In particular, the porous membranes or the multilayer porous membranesaccording to Embodiments barely make a short circuit at hightemperatures, and can be safely used as a separator for batteries with ahigh electromotive force.

Examples of the batteries with a high electromotive force includenonaqueous electrolyte batteries. The nonaqueous electrolyte battery canbe produced by a standard method, for example, by disposing the porousmembrane or the multilayer porous membrane according to Embodimentsbetween a positive electrode and a negative electrode and allowing anonaqueous electrolytic solution to be held.

Any known positive electrode, negative electrode, and nonaqueouselectrolytic solution can be used without particular limitation.

Examples of positive electrode materials include lithium-containingcomplex oxides such as LiCoO₂, LiNiO₂, spinel type LiMnO₄,Li[Ni_(x)Mn_(y)Co_(z)]O₂ (where x, y, and z satisfy x+y+z=1 and 0≦x<1,0≦y<1, and 0≦z<1), and olivine type LiFePO₄. Examples of negativeelectrode materials include: carbon materials such as graphite,non-graphitizable carbon, graphitizable carbon, and composite carbon;silicon; tin; metal lithium; and a variety of alloy materials.

As the nonaqueous electrolytic solution, an electrolytic solutionprepared by dissolving an electrolyte in an organic solvent can be used.Examples of the organic solvent include propylene carbonate, ethylenecarbonate, dimethyl carbonate, diethyl carbonate, and ethyl methylcarbonate. Examples of the electrolyte include lithium salts such asLiClO₄, LiBF₄, and LiPF₆.

In the case where the porous membrane or the multilayer porous membraneis used as a separator for batteries, the air permeability of the porousmembrane or the multilayer porous membrane is preferably not less than10 seconds/100 cc and not more than 500 seconds/100 cc, more preferablynot less than 20 seconds/100 cc and not more than 400 seconds/100 cc,and still more preferably not less than 30 seconds/100 cc and not morethan 300 seconds/100 cc. At an air permeability of not less than 10seconds/100 cc, the self-discharge tends to be reduced, when used as aseparator for batteries. At an air permeability of not more than 500seconds/100 cc, good charging and discharging properties tend to beobtained.

The membrane thickness of the porous membrane or the multilayer porousmembrane is preferably not less than 2 μm and not more than 200 μm, morepreferably not less than 5 μm and not more than 100 μm, and still morepreferably not less than 7 μm and not more than 30 μm. At a membranethickness of not less than 2 μm, sufficient mechanical strength tends tobe obtained. At a membrane thickness of not more than 200 μm, the volumeoccupied by the separator tends to be reduced, which is advantageousfrom the viewpoint of increase in the battery capacity.

The thermal shrinkage rate at 150° C. of the porous membrane or themultilayer porous membrane is preferably not less than 0% and not morethan 15%, more preferably not less than 0% and not more than 10%, andstill more preferably not less than 0% and not more than 5%, both in theMD and in the TD. A thermal shrinkage rate of not more than 15% in theMD and the TD is preferred because breakage of the multilayer porousmembrane at the time of abnormal heat generation of the battery tends tobe suppressed, and hardly causing a short circuit.

The shutdown temperature of the porous membrane or the multilayer porousmembrane is preferably not less than 120° C. and not more than 160° C.,and more preferably not less than 120° C. and not more than 150° C. Ashutdown temperature of not more than 160° C. is preferred because rapidcurrent interruption tends to promote when the battery generates heat,providing higher safe performance. On the other hand, a shutdowntemperature of not less than 120° C. is preferred because the batterycan be used at around 100° C.

The shutdown temperature can be adjusted by controlling the kind of thepolyolefin resin and the composition ratio thereof, the stretchingtemperature, the stretch ratio, the heat setting temperature, thestretch ratio during heat setting, and the relaxation rate during heatsetting, and combining these.

The short-circuit temperature of the porous membrane or the multilayerporous membrane is preferably not less than 180° C. and not more than1000° C., and more preferably not less than 200° C. and not more than1000° C. At a short-circuit temperature of not less than 180° C., evenif abnormal heat generation occurs in the battery, a short circuit doesnot immediately occur. Accordingly, the heat can be dissipated duringthat period, and higher safe performance is obtained.

The short-circuit temperature can be controlled at a desired value byadjusting the content of polypropylene, the kind of polyolefin otherthan polypropylene, the kind of inorganic fillers, the thickness of theinorganic filler containing porous layer, and the like.

The various parameters mentioned above are measured according tomeasuring methods in Examples described later, unless otherwisespecified.

EXAMPLES

Hereinafter, Embodiments will be described in more detail. Here,Embodiments are not limited to the following Examples as long as it doesnot go beyond the gist thereof.

Examples 1 to 11 below correspond to the examples for Embodiment 1.

In Examples, physical properties were determined by the followingmethods. If the atmosphere for the measurement is not specified, themeasurement was performed in the air at 23° C. and a pressure of 1 atm.

(1) Viscosity Average Molecular Weight (Mv) of Polyolefin

The limiting viscosity [η] (dl/g) at 135° C. in a decalin solvent wasdetermined according to ASTM-D4020.

The Mv of Polyethylene was calculated by the following equation.

[η]=6.77×10⁻⁴ Mv^(0.67)

The Mv of polypropylene was calculated by the following equation.

[η]=1.10×10⁻⁴ Mv^(0.80)

(2) Membrane Thickness of Porous Membrane or Multilayer Porous Membrane,and Layer Thickness of Porous Layer

Samples of MD 10 mm×TD 10 mm were cut out from a porous membrane and amultilayer porous membrane. In each sample, nine points (threepoints×three points) positioned in a lattice shape were selected, andthe membrane thicknesses thereof were measured using a dial gauge (madeby Ozaki MFG. Co₁, Ltd., PEACOCK No. 25 (registered trademark)). Themembrane thickness (μm) of the porous membrane and that of themultilayer porous membrane each were defined as the average value of themeasured values at the nine points. The difference between the membranethickness of the multilayer porous membrane thus measured and that ofthe porous membrane thus measured was defined as the layer thickness(μm) of the porous layer.

(3) Porosity (%) of Porous Membrane

A sample of a 10 cm×10 cm square was cut out from the porous membrane,and the volume (cm³) and mass (g) thereof were determined. Applying amembrane density of 0.95 (g/cm³) for the porous membrane, the porositywas calculated using the following equation.

Porosity(%)=(1−mass/volume/0.95)×100

(4) Air Permeability of Porous Membrane and Multilayer Porous Membrane

Using a Gurley type densometer (made by Toyo Seiki Seisaku-sho, Ltd.,G-B2 (trademark), the mass of the internal cylinder: 567 g) according toJIS P-8117, a time (sec) taken for 100 cc of the air to pass through theporous membrane having an area of 645 mm² (a circle having a diameter of28.6 mm) and a time (sec) taken for 100 cc of the air to pass throughthe multilayer porous membrane having the same size were measured. Thesemeasured values were defined as the air permeability (sec/100 cc) of theporous membrane and that of the multilayer porous membrane,respectively.

(5) Average Particle Size of Inorganic Fillers

Inorganic fillers were added to distilled water, a small amount of anaqueous solution of sodium hexametaphosphate was added thereto, anddispersed for 1 minute by an ultrasonic homogenizer. Then, particle sizedistribution was measured using a laser particle size distributionanalyzer (made by Nikkiso Co., Ltd., Microtrac MT3300EX). The averageparticle size (μm) was defined as a particle size when a cumulativefrequency reached 50%.

(6) Average Pore Size, Tortuosity, and the Number of Pores of PorousMembrane Determined by Gas-Liquid Method

It is known that a fluid in a capillary flows according to the Knudsenflow when the mean free path of the fluid is larger than the pore sizeof the capillary, and flows according to the Poiseuille flow when themean free path of the fluid is smaller than the pore size of thecapillary. Thus, it is assumed that the air stream in the measurement ofthe air permeability of the porous membrane follows the Knudsen flow,and the water stream in the measurement of water permeability of theporous membrane follows the Poiseuille flow.

In this case, the average pore size d (μm) and the tortuosity τ_(a)(dimensionless) of the porous membrane were determined from the airpenetration rate constant R_(gas) (m³/(m²·sec·Pa)), the waterpenetration rate constant R_(liq) (m³/(m²·sec·Pa)), the velocity of theair molecule ν (m/sec), the viscosity of water η (Pa·sec), the standardpressure P_(s) (=101325 Pa), the porosity ε (%), and the membranethickness L (μm) using the following equations:

d=2ν×(R _(liq) /R _(gas))×(16η/3P _(s))×10⁶

τ_(a)=(d×(ε/100)×ν/(3L×P _(s) ×R _(gas)))^(1/2)

R_(gas) was determined from the air permeability (sec) using thefollowing equation:

R _(gas)=0.0001/(air permeability×(6.424×10⁻⁴)×(0.01276×101325))

R_(liq) was determined from the water permeability (cm³/(cm²·sec·Pa))using the following equation:

R _(liq)=water permeability/100

The water permeability was determined as follows. A porous membraneimmersed in ethanol in advance was set in a stainless steel liquidpermeation cell having a diameter of 41 mm. After the ethanol adheringto the membrane was washed with water, water was permeated into themembrane at a differential pressure of approximately 50000 Pa. From theamount of water permeated (cm³) when 120 sec had passed, the amount ofwater permeated per unit time·unit pressure·unit area was calculated,and was defined as the water permeability.

ν was determined from a gas constant R (=8.314), the absolutetemperature T(K), the circular constant π, and the average molecularweight M of air (=2.896×10⁻² kg/mol) using the following equation:

ν=((8R×T)/(π×M))^(1/2)

The number B of pores (pores/μm²) was determined from the followingequation:

B=4×(ε/100)/(π×d ²×τ_(a))

(7) Puncture Strength of Porous Membrane

A porous membrane was fixed with a sample holder having an opening witha diameter of 11.3 mm, using a handy compression tester KES-G5(trademark) made by Kato tech Co., Ltd. Next, the center of the fixedporous membrane was subjected to a puncture test with a needle having atip with a curvature radius of 0.5 mm at a puncture rate of 2 mm/secunder a 25° C. atmosphere to determine the largest puncture load (gf).The value was multiplied by 25/membrane thickness (μm) to calculate thepuncture strength (gf/25 μm) in terms of the membrane thickness of 25μm.

(8) Membrane Resistance of Porous Membrane

A cut sample having a size of 2.6 cm×2.0 cm was prepared, and wasimmersed in a methanol solution having 3% by mass nonionic surfactant(made by Kao Corporation, EMULGEN 210P) dissolved therein. The samplewas air dried. An aluminum foil having a thickness of 20 μm was cut intoa size of 2.0 cm×1.4 cm, and a lead tab was attached to the foil. Twoaluminum foils were thus prepared, and the cut sample was sandwichedbetween the aluminum foils so as to avoid short circuit of the aluminumfoil. The sample was impregnated with an electrolytic solution, 1 MLiBF₄ propylene carbonate/ethylene carbonate (1/1 in mass ratio). Thesample was sealed under reduced pressure in an aluminum laminate packsuch that the tabs were projected from the aluminum laminate pack to theoutside. Such cells were prepared such that the number of porousmembranes in the aluminum foil was one sheet, two sheets, and threesheets. Each of the cells was placed in a 20° C. thermostat, and theresistance of the cell was measured by an alternating current impedancemethod at an amplitude of 10 mV and a frequency of 100 kHz. The measuredresistance value of the cell was plotted against the number of sheet ofthe porous membranes. The plot was linearly approximated to determinethe slope. The slope was multiplied by the area of an electrode, thatis, 2.0 cm×1.4 cm to determine the membrane resistance R (Ω·cm²) persheet of the porous membrane.

(9) Tortuosity of Porous Membrane Determined from Membrane Resistance

From the membrane resistance R (Ω·cm²) and the porosity ε (%) of theporous membrane, the specific resistance ρ (Ω·cm) of the electrolyticsolution, and the membrane thickness L (μm) of the porous membrane, thetortuosity of porous membrane was determined using the followingequation:

τ_(b)={(R·ε)/(ρ·L)}^((1/2))

where the electrolytic solution used was a 1 M LiBF₄ propylenecarbonate/ethylene carbonate (1/1 in mass ratio) (made by KishidaChemical Co., Ltd.) at 20° C.; in this case, ρ was 2.663×10⁻² Ω·cm.

(10) Withstand Voltage of Porous Membrane and Multilayer Porous Membrane

A porous membrane or a multilayer porous membrane was sandwiched betweenaluminum electrodes having a diameter of 4 cm, and a load of 15 g wasapplied thereon. The laminate was connected to a withstand voltagemeasuring apparatus (TOS9201) made by KIKUSUI ELECTRONICS CORPORATION.For the measurement conditions, an AC voltage (60 Hz) was applied at arate of 1.0 kV/sec, and the voltage value when a short circuit occurredwas defined as the withstand voltage (kV) of the porous membrane or themultilayer porous membrane.

(11) Average Particle Size of Resin Binder

The volume average particle size (nm) was measured using a particle sizemeasuring apparatus (MICROTRACTMUPA150 made by LEED & NORTHRUP Company)according to a light scattering method to define the obtained value asthe average particle size.

(12) Thermal Shrinkage Rate (%) at 150° C.

A separator was cut into 100 mm in the MD and 100 mm in the TD, and leftas it was in a 150° C. oven for 1 hour. At this time, the sample wassandwiched between two sheets of paper so as not to be exposed to warmair directly. The sample was taken out from the oven to be cooled. Thelength (mm) of the sample was measured, and the thermal shrinkage ratesin the MD and in the TD were calculated by the following equations.

MD thermal shrinkage rate(%)=(100−length of MD after heating)/100×100

TD thermal shrinkage rate(%)=(100−length of TD after heating)/100×100

(13) Shutdown Temperature and Short-Circuit Temperature of MultilayerPorous Membrane

a. Production of Positive Electrode

92.2 parts by mass of lithium cobalt complex oxide (LiCoO₂) as apositive electrode active substance, 2.3 parts by mass of flake graphiteand 2.3 parts by mass of acetylene black as an electric conductionmaterial, and 3.2 parts by mass of polyvinylidene fluorides (PVDF) as aresin binder were provided. These were dispersed in N-methylpyrrolidone(NMP) to prepare a slurry. Using a die coater, this slurry was appliedonto one surface of an aluminum foil having a thickness of 20 μm andserving as a positive electrode collector such that the amount of thepositive electrode active substance to be applied was 250 g/m². Afterdrying at 130° C. for 3 minutes, using a roll press machine, the productwas compression-formed such that the bulk density of the positiveelectrode active substance was 3.00 g/cm³. Thus, a positive electrodewas obtained.

b. Production of Negative Electrode

96.6 parts by mass of artificial graphite as a negative electrode activesubstance, and 1.4 parts by mass of an ammonium salt of carboxymethylcellulose and 1.7 parts by mass of a styrene-butadiene copolymer latexas a resin binder were provided. These were dispersed in purified waterto prepare a slurry. Using a die coater, this slurry was applied ontoone surface of a copper foil having a thickness of 12 μm and serving asa negative electrode collector such that the amount of the negativeelectrode active substance to be applied was 106 g/m². After drying at120° C. for 3 minutes, using a roll press machine, the product wascompression-formed such that the bulk density of the negative electrodeactive substance was 1.35 g/cm³. Thus, a negative electrode wasobtained.

c. Preparation of Nonaqueous Electrolytic Solution

LiBF₄ as a solute was dissolved at a concentration of 1.0 mol/L in amixed solvent of propylene carbonate:ethylenecarbonate:γ-butyllactone=1:1:2 (volume ratio), to prepare a nonaqueouselectrolytic solution.

d. Measurement of Shutdown Temperature and Short-Circuit Temperature

A negative electrode cut into 65 mm×20 mm and immersed in the nonaqueouselectrolytic solution for not less than 1 minute, an aramid film of 9 μm(thickness)×50 mm×50 mm having a hole with a diameter 16 mm in thecentral portion thereof, a multilayer porous membrane or porous membranecut into 65 mm×20 mm and immersed in the nonaqueous electrolyticsolution for not less than 1 hour, a positive electrode cut into 65mm×20 mm and immersed in the nonaqueous electrolytic solution for notless than 1 minute, a Kapton film, and a silicone rubber having athickness of approximately 4 mm were provided, and laminated in theabove order on a ceramic plate having a thermocouple connected thereto.This laminated body was set on a hot plate. While a pressure of 4.1 MPawas applied to the laminated body by a hydraulic press machine, thetemperature was raised at the rate of 15° C./min. Change in theimpedance between the positive electrode and the negative electrode wasmeasured to 200° C. under the conditions of a 1 V and 1 kHz alternatingcurrent.

The shutdown temperature was defined as a temperature at which theimpedance reached 1000Ω. The short-circuit temperature was defined as atemperature at which the impedance fell below 1000Ω again aftershutdown.

(14) Rate Capability of Multilayer Porous Membrane

a. Production of Positive Electrode

91.2 Parts by mass of lithium nickel manganese cobalt complex oxide(Li[Ni_(1/3)Mn_(1/3)Co_(1/3)]O₂) as a positive electrode activesubstance, 2.3 parts by mass of flake graphite and 2.3 parts by mass ofacetylene black as electric conductive materials, and 4.2 parts by massof polyvinylidene fluoride (PVDF) as a resin binder were provided. Thesewere dispersed in N-methyl pyrrolidone (NMP) to prepare a slurry. Theslurry was applied onto one surface of an aluminum foil having athickness of 20 μm and serving as a positive electrode collector with adie coater such that the amount of the positive electrode activesubstance applied was 120 g/m². The coating was dried at 130° C. for 3minutes. The product was then compression-formed with a roll press suchthat the bulk density of the positive electrode active substance was2.90 g/cm². A positive electrode was prepared. Thus, a positiveelectrode was punched into a circle having an area of 2.00 cm².

b. Production of Negative Electrode

96.6 Parts by mass of artificial graphite as a negative electrode activesubstance, and 1.4 parts by mass of ammonium salt of carboxymethylcellulose and 1.7 parts by mass of a styrene-butadiene copolymer latexas resin binders were provided. These were dispersed in purified waterto prepare a slurry. The slurry was applied onto one surface of a copperfoil having a thickness of 16 μm and serving as a negative electrodecollector with a die coater such that the amount of the negativeelectrode active substance applied was 53 g/m². After drying at 120° C.for 3 minutes, the product was then compression-formed with a roll presssuch that the bulk density of the negative electrode active substancewas 1.35 g/cm². Thus, a negative electrode was prepared. The negativeelectrode was punched into a circle having an area of 2.05 cm².

c. Nonaqueous Electrolytic Solution

LiPF₆ as a solute was dissolved at a concentration of 1.0 ml/L in amixed solvent of ethylene carbonate:ethylmethyl carbonate=1:2 (volumeratio) to prepare a nonaqueous electrolytic solution.

d. Assembly of Battery

The negative electrode, the multilayer porous membrane, and the positiveelectrode were laminated in this order such that the active substancesurface of the positive electrode faces that of the negative electrode.This laminated body was placed in a stainless steel metal container witha cover such that the copper foil of the negative electrode and thealuminum foil of the positive electrode contact the main body of thecontainer and the cover, respectively. The main body of the container isinsulated from the cover of the container. The nonaqueous electrolyticsolution was injected into this container, and the container was sealed.

e. Evaluation of Rate Capability

A simple battery assembled in d. was charged to a cell voltage of 4.2 Vat a current value of 3 mA (approximately 0.5 C) at 25° C. Reduction ofthe current value was started from 3 mA such that 4.2 V was maintained.In such a manner, initial charging after production of the battery wasperformed for approximately 6 hours in total, and then, discharging wasperformed to a cell voltage of 3.0 V at a current value of 3 mA.

Next, charging was performed to a cell voltage of 4.2 V at a currentvalue of 6 mA (approximately 1.0 C) at 25° C. Reduction of the currentvalue was started from 6 mA such that 4.2 V was maintained. In such amanner, charging was performed for approximately 3 hours in total, andthen, discharging was performed to a cell voltage of 3.0 V at a currentvalue of 6 mA. The discharge capacity at the time was defined as 1 Cdischarge capacity (mAh).

Next, charging was performed to a cell voltage of 4.2 V at a currentvalue of 6 mA (approximately 1.0 C) at 25° C. Reduction of the currentvalue was started from 6 mA such that 4.2 V was maintained. In such amanner, charging was performed for approximately 3 hours in total, andthen, discharging was performed to a cell voltage of 3.0 V with atcurrent value of 12 mA (approximately 2 C). The discharge capacity atthe time was defined as 2 C discharge capacity (mAh).

Next, the battery was charged to a cell voltage of 4.2 V at 25° C. and acurrent value of 6 mA (approximately 1.0 C). Reduction of the currentvalue was started from 6 mA such that 4.2 V was maintained. In such amanner, the battery was charged for approximately 3 hours in total.Subsequently, the battery was discharged at a current value of 60 mA(approximately 10 C) to a cell voltage of 3.0 V. The discharge capacityat this time was defined as the 10 C discharge capacity (mAh).

The proportion of 2 C discharge capacity to 1 C discharge capacity wascalculated, and this value was defined as the rate capability at 2 C.

2C rate capability(%)=(2C discharge capacity/1C discharge capacity)×100

2C rate reduction rate(%)={(2C rate capability of porous membraneused)−(2C rate capability of multilayer porous membrane)}/(2C ratecapability of porous membrane used)×100

The proportion of a 10 C discharge capacity to a 1 C discharge capacitywas calculated, and the value was defined as the rate capability at 10C.

10C rate capability(%)=(10 C discharge capacity/1C dischargecapacity)×100

10C rate reduction rate(%)={(10C rate capability of porous membraneused)−(10C rate capability of multilayer porous membrane)}/(10C ratecapability of porous membrane used)×100

Example 1

47.5 parts by mass of a homopolymer polyethylene having an Mv of700,000, 47.5 parts by mass of a homopolymer polyethylene having an Mvof 250,000, and 5 parts by mass of a homopolymer polypropylene having anMv of 400,000 were dry blended using a tumbler blender. 1 part by massofpentaerythrityl-tetrakis-[3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate]was added as an antioxidant to 99 parts by mass of the obtained apolymer mixture, and dry blended again using the tumbler blender.Thereby, a mixture of the polymers and the like was obtained. Afterreplacement by nitrogen, the obtained mixture of the polymers and thelike was fed to a twin screw extruder by a feeder under a nitrogenatmosphere. Liquid paraffin (kinematic viscosity of 7.59×10⁻⁵ m²/s at37.78° C.) was injected into a cylinder of the extruder by a plungerpump.

The feeder and the pump were adjusted such that the ratio of the amountof liquid paraffin to the mixture of all the components melt kneaded andextruded was 67% by mass (resin composition content: 33% by mass). Forthe melt kneading conditions, a preset temperature was 200° C., a screwrotation speed was 100 rpm, and an amount of discharge was 12 kg/h.

Then, the melt kneaded product was extruded through a T die and castonto a cooling roll, whose surface temperature was controlled to be 25°C., to obtain a gel sheet having a thickness of 1600 μm.

Next, the gel sheet was guided to a simultaneous biaxial tenterstretching machine to be subjected to biaxial stretching. For thestretching condition settings, the MD magnification was 7.0 times, theTD magnification was 6.1 times, and a preset temperature was 121° C.

Next, the sheet was guided to a tank of methyl ethyl ketone, and wassufficiently immersed in methyl ethyl ketone to remove liquid paraffinby extraction. Subsequently methyl ethyl ketone was removed by drying.

Next, the sheet was guided to a TD tenter, and was subjected to heatsetting. The heat setting temperature was 120° C., the largest TDmagnification was 2.0 times, and the relaxation rate was 0.90. As aresult, a polyolefin resin porous membrane having: a membrane thicknessof 17 μm; a porosity of 60%; an air permeability of 84 seconds/100 cc;an average pore size d=0.057 μm, a tortuosity τa=1.45, and the number Bof pores=165 pores/μm², which were determined by the gas-liquid method;and a puncture strength of 567 gf in terms of 25 μm, was obtained.

Next, 95.0 parts by mass of calcined kaolin prepared by calcining wetkaolin (kaolinite (Al₂Si₂O₅(OH)₄) as a main component) at a hightemperature (average particle size: 1.8 μm), 5.0 parts by mass of anacrylic latex (solid content: 40%, average particle size: 220 nm, lowestmembrane forming temperature: 0° C. or less), and 0.5 parts by mass ofan aqueous solution of ammonium polycarboxylate (SN-DISPERSANT 5468 madeby SAN NOPCO Limited) were uniformly dispersed in 180 parts by mass ofwater to prepare a coating solution. The coating solution was appliedonto the surface of the polyolefin resin porous membrane with a gravurecoater. Water was removed by drying at 60° C. to obtain a multilayerporous membrane including the polyolefin resin porous membrane and aporous layer having a thickness of 7 μm and formed on the polyolefinresin porous membrane.

Example 2

A polyolefin resin porous membrane having: a membrane thickness of 18μm; a porosity of 64%; an air permeability 78 seconds/100 cc; an averagepore size d=0.055 μm, tortuosity τa=1.38, and the number B of pores=191pores/μm², which were determined by the gas-liquid method; and apuncture strength of 542 gf in terms of 25 μm, was obtained in the samemanner as that in Example 1 except that the melt kneaded product wascast through the T die to obtain a gel sheet having a thickness of 1550μm, and a preset temperature for biaxial stretching with thesimultaneous biaxial tenter stretching machine was 119° C.

Next, 96.0 parts by mass of aluminum hydroxide oxide (average particlesize: 1.0 μm), 4.0 parts by mass of an acrylic latex (solid content:40%, average particle size: 145 nm, lowest membrane forming temperature:0° C. or less), and 1.0 part by mass of an aqueous solution of ammoniumpolycarboxylate (SN-DISPERSANT 5468 made by SAN NOPCO Limited) wereuniformly dispersed in 100 parts by mass of water to prepare a coatingsolution. The coating solution was applied to the surface of thepolyolefin resin porous membrane with a micro gravure coater. Themembrane was dried at 60° C. to remove water to obtain a multilayerporous membrane including the polyolefin resin porous membrane and aporous layer having a thickness of 7 μm formed on the polyolefin resinporous membrane.

Example 3

A polyolefin resin porous membrane having: a membrane thickness of 15μm; a porosity of 60%; an air permeability of 90 seconds/100 cc; anaverage pore size d=0.056 μm, a tortuosity τa=1.54, and the number B ofpores=157 pores/μm², which were determined by the gas-liquid method; anda puncture strength of 600 gf in terms of 25 μm, was obtained in thesame manner as that in Example 1 except that the melt kneaded productwas cast through the T die to obtain a gel sheet having a thickness of1400 μm, a preset temperature for biaxial stretching with thesimultaneous biaxial tenter stretching machine was 119° C., the heatsetting temperature in the TD tenter was 128° C., the largest TDmagnification was 2.0 times, and the relaxation rate was 0.88.

Next, 95.0 parts by mass of calcined kaolin prepared by calcining wetkaolin(kaolinite (Al₂Si₂O₅(OH)₄) as a main component) at a hightemperature (average particle size: 1.8 μm), 5.0 parts by mass of anacrylic latex (solid content: 40%, average particle size: 145 nm, lowestmembrane forming temperature: 0° C. or less), and 0.5 parts by mass ofan aqueous solution of ammonium polycarboxylate (SN-DISPERSANT 5468 madeby SAN NOPCO Limited) were uniformly dispersed in 180 parts by mass ofwater to prepare a coating solution. The coating solution was appliedonto the surface of the polyolefin resin porous membrane with a microgravure coater. Water was removed by drying at 60° C. to obtain amultilayer porous membrane including the polyolefin resin porousmembrane and a porous layer having a thickness of 6 μm and formed on thepolyolefin resin porous membrane.

Example 4

A polyolefin resin porous membrane having: a membrane thickness of 13μm; a porosity of 64%; an air permeability 65 seconds/100 cc; an averagepore size d=0.050 μm, tortuosity τa=1.41, and the number B of pores=222pores/μm², which were determined by the gas-liquid method; and apuncture strength of 618 gf in terms of 25 μm, was obtained in the samemanner as that in Example 1 except that the melt kneaded product wascast through the T die to obtain a gel sheet having a thickness of 1150μm, and a preset temperature for biaxial stretching with thesimultaneous biaxial tenter stretching machine was 120° C.

Next, 95.0 parts by mass of calcined kaolin prepared by calcining wetkaolin (kaolinite (Al₂Si₂O₅(OH)₄) as a main component) at a hightemperature (average particle size: 1.1 μm), 5.0 parts by mass of anacrylic latex (solid content: 40%, average particle size: 145 nm, lowestmembrane forming temperature: 0° C. or less), and 0.5 parts by mass ofan aqueous solution of ammonium polycarboxylate (SN-DISPERSANT 5468 madeby SAN NOPCO Limited) were uniformly dispersed in 180 parts by mass ofwater to prepare a coating solution. The coating solution was appliedonto the surface of the polyolefin resin porous membrane with a microgravure coater. Water was removed by drying at 60° C. to obtain amultilayer porous membrane including the polyolefin resin porousmembrane and a porous layer having a thickness of 5 μm and formed on thepolyolefin resin porous membrane.

Example 5

A polyolefin resin porous membrane having: a membrane thickness of 17μm; a porosity of 57%; an air permeability of 132 seconds/100 cc; anaverage pore size d=0.052 μm, a tortuosity τa=1.64, and the number B ofpores=163 pores/μm², which were determined by the gas-liquid method; anda puncture strength of 788 gf in terms of 25 μm, was obtained in thesame manner as that in Example 1 except that the melt kneaded productwas cast through the T die to obtain a gel sheet having a thickness of1700 μm, a preset temperature for biaxial stretching with thesimultaneous biaxial tenter stretching machine was 117° C., the heatsetting temperature in the TD tenter was 117° C.

Next, 96.0 parts by mass of calcined kaolin prepared by calcining wetkaolin (kaolinite (Al₂Si₂O₅(OH)₄) as a main component) at a hightemperature (average particle size: 1.1 μm), 4.0 parts by mass of anacrylic latex (solid content: 40%, average particle size: 145 nm, lowestmembrane forming temperature: 0° C. or less), and 1.0 part by mass of anaqueous solution of ammonium polycarboxylate (SN-DISPERSANT 5468 made bySAN NOPCO Limited) were uniformly dispersed in 100 parts by mass ofwater to prepare a coating solution. The coating solution was appliedonto the surface of the polyolefin resin porous membrane with a gravurecoater. Water was removed by drying at 60° C. to obtain a multilayerporous membrane including the polyolefin resin porous membrane and aporous layer having a thickness of 7 μm and formed on the polyolefinresin porous membrane.

Example 6

A polyolefin resin porous membrane having: a membrane thickness of 12μm; a porosity of 65%; an air permeability of 61 seconds/100 cc; anaverage pore size d=0.043 μm, a tortuosity τa=1.36, and the number B ofpores=325 pores/μm², which were determined by the gas-liquid method; anda puncture strength of 678 gf in terms of 25 μm, was obtained in thesame manner as that in Example 1 except that the melt kneaded product(the ratio of the amount of liquid paraffin to the mixture of all thecomponents melt kneaded and extruded was 68% by mass; resin compositioncontent: 32% by mass) was cast through the T die to obtain a gel sheethaving a thickness of 1050 μm, a preset temperature for biaxialstretching with the simultaneous biaxial tenter stretching machine was120° C., and the heat setting temperature in the TD tenter was 119° C.

Next, 96.5 parts by mass of aluminum oxide (average particle size: 1.0μm), and 3.5 parts by mass of an acrylic latex (solid content: 40%,average particle size: 145 nm, lowest membrane forming temperature: 0°C. or less) were uniformly dispersed in 150 parts by mass of water toprepare a coating solution. The coating solution was applied onto thesurface of the polyolefin resin porous membrane with a micro gravurecoater. Water was removed by drying at 60° C. to obtain a multilayerporous membrane including a polyolefin resin porous membrane and aporous layer having a thickness of 6 μm and formed on the polyolefinresin porous membrane.

Example 7

95 parts by mass of a homopolymer polyethylene having an Mv of 250,000and 5 parts by mass of a homopolymer polyethylene having an Mv of400,000 were dry blended using a tumbler blender. 1 part by mass ofpentaerythrityl-tetrakis-[3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate]as an antioxidant was added to 99 parts by mass of the obtained polymermixture, and dry blended again using the tumbler blender. Thereby, amixture of the polymers and the like was obtained. After replacement bynitrogen, the obtained mixture of the polymers and the like was fed to atwin screw extruder by a feeder under a nitrogen atmosphere. Liquidparaffin (kinematic viscosity of 7.59×10⁻⁵ m²/s at 37.78° C.) wasinjected into a cylinder of the extruder by a plunger pump.

The feeder and the pump were adjusted such that the ratio of the amountof liquid paraffin to the mixture of all the components melt kneaded andextruded was 62% by mass (resin composition content: 38% by mass). Forthe melt kneading conditions, a preset temperature was 200° C., a screwrotation speed was 100 rpm, and an amount of discharge was 12 kg/h.

Then, the melt kneaded product was extruded through a T die and castonto a cooling roll, whose surface temperature was controlled to be 25°C., to obtain a gel sheet having a thickness of 1200 μm.

Next, the gel sheet was guided to a simultaneous biaxial tenterstretching machine to be subjected to biaxial stretching. For thestretching condition settings, the MD magnification was 7.0 times, theTD magnification was 5.0 times, and a preset temperature was 123° C.

Next, the sheet was guided to a tank of methyl ethyl ketone, and wassufficiently immersed in methyl ethyl ketone to remove liquid paraffinby extraction. Subsequently methyl ethyl ketone was removed by drying.

Next, the sheet was guided to a TD tenter, and was subjected to heatsetting. The heat setting temperature was 125° C., the largest TDmagnification was 3.5 times, and the relaxation rate was 0.94. As aresult, a polyolefin resin porous membrane having: a membrane thicknessof 11 μm; a porosity of 67%; an air permeability of 40 seconds/100 cc;an average pore size d=0.056 μm, a tortuosity τa=1.25, and the number Bof pores=223 pores/μm², which were determined by the gas-liquid method;and a puncture strength of 658 gf in terms of 25 μm, was obtained.

Next, 96.0 parts by mass of aluminum hydroxide oxide (average particlesize: 1.0 μm), 4.0 parts by mass of an acrylic latex (solid content:40%, average particle size: 140 nm, lowest membrane forming temperature:0° C. or less), and 0.5 parts by mass of an aqueous solution of ammoniumpolycarboxylate (SN-DISPERSANT 5468 made by SAN NOPCO Limited) wereuniformly dispersed in 180 parts by mass of water to prepare a coatingsolution. The coating solution was applied onto the surface of thepolyolefin resin porous membrane with a gravure coater. The membrane wasdried at 60° C. to remove water to obtain a multilayer porous membraneincluding the polyolefin resin porous membrane and a porous layer havinga thickness of 5 μm formed on the polyolefin resin porous membrane.

Example 8

96.0 Parts by mass of aluminum hydroxide oxide (average particle size:1.0 μm), 4.0 parts by mass of an acrylic latex (solid content: 40%,average particle size: 140 nm, lowest membrane forming temperature: 0°C. or less), and 0.5 parts by mass of an aqueous solution of ammoniumpolycarboxylate (SN-DISPERSANT 5468 made by SAN NOPCO Limited) wereuniformly dispersed in 180 parts by mass of water to prepare a coatingsolution. The coating solution was applied onto one surface of thepolyolefin resin porous membrane obtained in Example 6 with a microgravure coater, and was dried at 60° C. to remove water. The coatingsolution was then applied onto the other surface of the membrane withthe micro gravure coater. Water was removed by drying at 60° C. toobtain a multilayer porous membrane including a polyolefin resin porousmembrane and porous layers having a thickness of 6 μm and formed on bothsurfaces of the polyolefin resin porous membrane.

Example 9

A multilayer porous membrane was obtained by the same method as that inExample 1 except that a micro gravure coater and a dryer were arrangedin series immediately after the heat setting apparatus, the polyolefinresin porous membrane after heat setting was directly guided to themicro gravure coater without winding the membrane, and the coatingsolution was applied onto the surface of the polyolefin porous membrane,and was dried.

Example 10

28.5 Parts by mass of copolymerized polyethylene having an My of 150000(comonomer:propylene, content of propylene monomer unit: 0.6 mol %,density: 0.95), 28.5 parts by mass of high density homopolyethylenehaving an Mv of 300000, 14.2 parts by mass of high densityhomopolyethylene having an Mv of 700000, 23.8 parts by mass of ultrahigh molecular weight homopolyethylene having an Mv of 2000000, and 5parts by mass of homopolymer polypropylene were dry blended with atumbler blender.

As an antioxidant, 1 part by mass ofpentaerythrityl-tetrakis-[3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate]was added to 99 parts by mass of the obtained polymer mixture, and wasdry blended again with the tumbler blender to obtain a mixture of thepolymers and the like. After replacement by nitrogen, the obtainedmixture of the polymers and the like was fed to a twin screw extruderunder a nitrogen atmosphere with a feeder. Liquid paraffin (kinematicviscosity at 37.78° C.: 7.59×10⁻⁵ m²/s) was injected into an extrudercylinder with a plunger pump.

The feeder and the pump were adjusted such that the ratio of the amountof liquid paraffin to the mixture of all the components melt kneaded andextruded was 62% by mass (resin composition content: 38% by mass). Forthe melt kneading conditions, a preset temperature was 200° C., a screwrotation speed was 100 rpm, and an amount of discharge was 12 kg/h.

Then, the melt kneaded product was extruded through a T die and castonto a cooling roll, whose surface temperature was controlled to be 25°C., to obtain a gel sheet having a thickness of 1600 μm.

Next, the gel sheet was guided to a simultaneous biaxial tenterstretching machine to be subjected to biaxial stretching. For thestretching condition settings, the MD magnification was 7.0 times, theTD magnification was 6.1 times, and a preset temperature was 123° C.

Next, the sheet was guided to a tank of methyl ethyl ketone, and wassufficiently immersed in methyl ethyl ketone to remove liquid paraffinby extraction. Subsequently methyl ethyl ketone was removed by drying.

Next, the sheet was guided to a TD tenter, and subjected to heatsetting. The heat setting temperature was 117° C., the largest TDmagnification was 2.0 times, and the relaxation rate was 0.90. As aresult, a polyolefin resin porous membrane having: a membrane thicknessof 18 μm; a porosity of 57%; an air permeability of 116 seconds/100 cc;an average pore size d=0.057 μm, a tortuosity τa=1.61, and the number Bof pores=138 pores/μm², which were determined by the gas-liquid method;and a puncture strength of 506 gf in terms of 25 μm, was obtained.

The same porous layer as that in Example 1 was formed on the surface ofthe polyolefin resin porous membrane to obtain a multilayer porousmembrane.

Example 11

96.0 parts by mass of aluminum hydroxide oxide (average particle size:1.0 μm), 4.0 parts by mass of an acrylic latex (solid content: 40%,average particle size: 140 nm, lowest membrane forming temperature: 0°C. or less), and 0.5 parts by mass of an aqueous solution of ammoniumpolycarboxylate (SN-DISPERSANT 5468 made by SAN NOPCO Limited) wereuniformly dispersed in 180 parts by mass of water to prepare a coatingsolution. The coating solution was applied onto the surface of thepolyolefin resin porous membrane obtained in Example 10 with a gravurecoater. The membrane was dried at 60° C. to remove water to obtain amultilayer porous membrane including the polyolefin resin porousmembrane and a porous layer having a thickness of 6 μm formed on thepolyolefin resin porous membrane.

Comparative Example 1

40.5 parts by mass of high density polyethylene (weight averagemolecular weight: 250000, molecular weight distribution: 7, density:0.956), 4.5 parts by mass of linear copolymerized polyethylene (meltindex: 0.017, density: 0.930, propylene content: 1.6 mol %), 55 parts bymass of liquid paraffin, and 0.3 parts by mass (based on thepolyethylene) oftetrakis-[methylene-3-(3′,5′-di-t-butyl-4′-hydroxyphenyl)propionate]methanewere kneaded with a biaxial extruder to prepare a polymer solution. Theobtained polymer solution was cast from a hanger coat die having a lipclearance of 1.8 mm to a cooling roll to obtain a sheet having athickness of 1.8 mm. The obtained sheet was stretched 7×4 times beforeextraction with a simultaneous biaxial tenter stretching machine at astretching temperature of 120° C., and subsequently was immersed inmethylene chloride to remove liquid paraffin by extraction. The sheetwas stretched after extraction 2.8 times in the traverse direction at astretching temperature of 110° C. with a tenter stretching machine. Thenwhile the stretching in the traverse direction was 35% relaxed, thesheet was subjected to a heat treatment. As a result, a polyolefin resinporous membrane having: a membrane thickness of 26 μm; a porosity of65%; an air permeability of 75 seconds/100 cc; an average pore sized=0.060 μm, a tortuosity τa=1.18, and the number B of pores=195pores/μm², which were determined by the gas-liquid method; and apuncture strength of 339 gf in terms of 25 μm, was obtained.

The same porous layer as that in Example 2 was formed on the surface ofthe polyolefin resin porous membrane to obtain a multilayer porousmembrane.

Comparative Example 2

A polyolefin resin porous membrane having: a membrane thickness of 30μm; a porosity of 57%; an air permeability of 172 seconds/100 cc; anaverage pore size d=0.050 μm, a tortuosity τa=1.43, and the number B ofpores=203 pores/μm², which were determined by the gas-liquid method; anda puncture strength of 415 gf in terms of 25 μm, was obtained in thesame manner as that in Comparative Example 1 except that 28 parts bymass of high density polyethylene (weight average molecular weight:250000, molecular weight distribution: 7, density: 0.956), 12 parts bymass of linear copolymerized polyethylene (melt index: 0.017, density:0.930, propylene content: 1.6 mol %), 60 parts by mass of liquidparaffin, and 0.3 parts by mass (based on the polyethylene) oftetrakis-[methylene-3-(3′,5′-di-t-butyl-4′-hydroxyphenyl)propionate]methanewere stretched before extraction 7×7 times, and stretched afterextraction 1.87 times, and the relaxation rate was 10%.

The same porous layer as that in Example 2 was formed on the surface ofthe polyolefin resin porous membrane to obtain a multilayer porousmembrane.

Comparative Example 3

47.5 parts by mass of homopolymer polyethylene having an Mv of 700000,47.5 parts by mass of homopolymer polyethylene having an Mv of 250000,and 5 parts by mass of homopolymer polypropylene having an My of 400000were dry blended with a tumbler blender. As an antioxidant, 1 part bymass ofpentaerythrityl-tetrakis-[3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate]was added to 99 parts by mass of the obtained polymer mixture, and wasdry blended again with the tumbler blender to obtain a mixture of thepolymers and the like. After replacement by nitrogen, the obtainedmixture of the polymers and the like was fed to a twin screw extruderunder a nitrogen atmosphere with a feeder. As a plasticizer, liquidparaffin (kinematic viscosity at 37.78° C.: 7.59×10⁻⁵ m²/s) was injectedinto an extruder cylinder with a plunger pump. The feeder and the pumpwere adjusted such that the ratio of the amount of liquid paraffin tothe mixture of all the components melt kneaded and extruded was 65% bymass. For the melt kneading conditions, a preset temperature was 200°C., a screw rotation speed was 240 rpm, and an amount of discharge was12 kg/h.

Then, the melt kneaded product was extruded through a T die and castonto a cooling roll, whose surface temperature was controlled to be 25°C., to obtain a sheet-like polyolefin composition having a thickness of1300 μm.

Next, the sheet was guided to a simultaneous biaxial tenter stretchingmachine, and was subjected to simultaneous biaxial stretching of 7 timesin MD and 6.4 times in TD. At this time, a preset temperature in thesimultaneous biaxial tenter was 118° C. Next, the sheet was guided to atank of methyl ethyl ketone to remove liquid paraffin by extraction.Thereafter, methyl ethyl ketone was removed by drying.

Next, the sheet was guided to a TD tenter, and subjected to heatsetting. The heat setting temperature was 122° C., the largest TDmagnification was 1.4 times, and the relaxation rate was 0.85. As aresult, a polyolefin resin porous membrane having: a membrane thicknessof 16 μm; a porosity of 47%; an air permeability of 163 seconds/100 cc;an average pore size d=0.058 μm, a tortuosity τa=1.86, and the number Bof pores=91 pores/μm², which were determined by the gas-liquid method;and a puncture strength of 525 gf in terms of 25 μm, was obtained.

Next, 95.0 parts by mass of calcined kaolin prepared by calcining wetkaolin(kaolinite (Al₂Si₂O₅(OH)₄) as a main component) at a hightemperature (average particle size: 1.8 μm), 5.0 parts by mass of anacrylic latex (solid content: 40%, average particle size: 220 nm, lowestmembrane forming temperature: 0° C. or less), and 0.5 parts by mass ofan aqueous solution of ammonium polycarboxylate (SN-DISPERSANT 5468 madeby SAN NOPCO Limited) were uniformly dispersed in 180 parts by mass ofwater to prepare a coating solution. The coating solution was appliedonto the surface of the polyolefin resin porous membrane with a microgravure coater. Water was removed by drying at 60° C. to obtain amultilayer porous membrane including the polyolefin resin porousmembrane and a porous layer having a thickness of 7 μm and formed on thepolyolefin resin porous membrane.

Comparative Example 4

47 parts by mass of homopolymer polyethylene having an Mv of 700000, 46parts by mass of homopolymer polyethylene having an Mv of 250000, and 7parts by mass of homopolymer polypropylene having an Mv of 400000 weredry blended with a tumbler blender. As an antioxidant, 1 part by mass ofpentaerythrityl-tetrakis-[3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate]was added to 99 parts by mass of the obtained polymer mixture, and wasdry blended again with the tumbler blender to obtain a mixture of thepolymers and the like. After replacement by nitrogen, the obtainedmixture of the polymers and the like was fed to a twin screw extruderunder a nitrogen atmosphere with a feeder. As a plasticizer, liquidparaffin (kinematic viscosity at 37.78° C.: 7.59×10⁻⁵ m²/s) was injectedinto an extruder cylinder with a plunger pump. The feeder and the pumpwere adjusted such that the ratio of the amount of liquid paraffin tothe mixture of all the components melt kneaded and extruded was 65% bymass. For the melt kneading conditions, a preset temperature was 200°C., a screw rotation speed was 240 rpm, and an amount of discharge was12 kg/h.

Then, the melt kneaded product was extruded through a T die and castonto a cooling roll, whose surface temperature was controlled to be 25°C., to obtain a sheet-like polyolefin composition having a thickness of2000 μm.

Next, the sheet was guided to a simultaneous biaxial tenter stretchingmachine, and was subjected to simultaneous biaxial stretching of 7 timesin MD and 7 times in TD. At this time, a preset temperature in thesimultaneous biaxial tenter was 125° C. Next, the sheet was guided to atank of methyl ethyl ketone to remove liquid paraffin by extraction.Thereafter, methyl ethyl ketone was removed by drying.

Next, the sheet was guided to a TD tenter, and subjected to heatsetting. The heat setting temperature was 133° C., the largest TDmagnification was 1.9 times, and the relaxation rate was 0.84. As aresult, a polyolefin resin porous membrane having: a membrane thicknessof 16 μm; a porosity of 41%; an air permeability of 157 seconds/100 cc;an average pore size d=0.085 μm, a tortuosity τa=2.10, and the number Bof pores=36 pores/μm², which were determined by the gas-liquid method;and a puncture strength of 572 gf in terms of 25 μm, was obtained.

Next, 95.0 parts by mass of calcined kaolin prepared by calcining wetkaolin(kaolinite (Al₂Si₂O₅(OH)₄) as a main component) at a hightemperature (average particle size: 1.8 μm), 5.0 parts by mass of anacrylic latex (solid content: 40%, average particle size: 145 nm, lowestmembrane forming temperature: 0° C. or less), and 0.5 parts by mass ofan aqueous solution of ammonium polycarboxylate (SN-DISPERSANT 5468 madeby SAN NOPCO Limited) were uniformly dispersed in 180 parts by mass ofwater to prepare a coating solution. The coating solution was appliedonto the surface of the polyolefin resin porous membrane with a gravurecoater. Water was removed by drying at 60° C. to obtain a multilayerporous membrane including the polyolefin resin porous membrane and aporous layer having a thickness of 6 μm and formed on the polyolefinresin porous membrane.

Comparative Example 5

A polyolefin resin porous membrane having: a membrane thickness of 20μm; a porosity of 41%; an air permeability of 283 seconds/100 cc; anaverage pore size d=0.069 μm, a tortuosity τa=2.29, and the number B ofpores=50 pores/μm², which were determined by the gas-liquid method; anda puncture strength 760 gf in terms of 25 μm, was obtained in the samemanner as that in Comparative Example 4 except that the heat settingtemperature in the TD tenter was 125° C., the largest TD magnificationwas 1.6 times, and the relaxation rate was 0.80.

The same porous layer as that in Comparative Example 3 was formed on thesurface of the polyolefin resin porous membrane to obtain a multilayerporous membrane.

Comparative Example 6

A porous membrane was obtained in the same manner as that in Example 1except that no porous layer was formed.

Comparative Example 7

A porous membrane was obtained in the same manner as that in Example 2except that no porous layer was formed.

Comparative Example 8

A porous membrane was obtained in the same manner as that in Example 3except that no porous layer was formed.

Comparative Example 9

A porous membrane was obtained in the same manner as that in Example 4except that no porous layer was formed.

Comparative Example 10

A porous membrane was obtained in the same manner as that in Example 5except that no porous layer was formed.

Comparative Example 11

A porous membrane was obtained in the same manner as that in Example 6except that no porous layer was formed.

Comparative Example 12

A porous membrane was obtained in the same manner as that in Example 7except that no porous layer was formed.

Comparative Example 13

A porous membrane was obtained in the same manner as that in ComparativeExample 1 except that no porous layer was formed.

Comparative Example 14

A porous membrane was obtained in the same manner as that in ComparativeExample 2 except that no porous layer was formed.

Comparative Example 15

A porous membrane was obtained in the same manner as that in ComparativeExample 3 except that no porous layer was formed.

Comparative Example 16

A porous membrane was obtained in the same manner as that in ComparativeExample 4 except that no porous layer was formed.

Comparative Example 17

A porous membrane was obtained in the same manner as that in ComparativeExample 5 except that no porous layer was formed.

Comparative Example 18

A porous membrane was obtained in the same manner as that in Example 10except that no porous layer was formed.

The physical properties and the results of evaluation of the porousmembranes, the porous layers, and the multilayer porous membranes inExamples and Comparative Examples are shown in Table 1 and Table 2below.

TABLE 1 Porous membrane Tortu- osity deter- Puncture mined strength Mem-from Mem- Num- in brane mem- brane Air Aver- ber terms resis- With-brane Porous layer thick- Poros- permea- age Tortu- B of of tance standresis- Layer ness ity bility pore osity pores Puncture 25 μm R volt-tance Inorganic thick- L ε sec/ size d τa pores/ strength gf/ Ω · age τbfiller Binder ness μm % 100 cc μm — μm² gf 25 μm cm² kV — — — μm Example1 17 60 84 0.057 1.45 165 385 567 1.0 1.3 1.15 Calcined LTX 7 kaolinExample 2 18 64 78 0.055 1.38 191 399 542 0.9 1.4 1.08 Alumi- LTX 7 numhydrox- ide oxide Example 3 15 60 90 0.056 1.54 157 367 600 0.9 1.3 1.15Calcined LTX 6 kaolin Example 4 13 64 65 0.050 1.41 222 331 618 0.7 1.21.11 Calcined LTX 5 kaolin Example 5 17 57 132 0.052 1.64 163 525 7881.1 1.4 1.19 Calcined LTX 7 kaolin Example 6 12 65 61 0.043 1.36 325 328678 0.7 0.9 1.18 Alumi- LTX 6 num oxide Example 7 11 67 40 0.056 1.25223 300 658 0.6 0.9 1.14 Alumi- LTX 5 num hydrox- ide oxide Example 8 1265 61 0.043 1.36 325 328 678 0.7 0.9 1.18 Calcined LTX 6 + 6 kaolinExample 9 17 60 84 0.057 1.45 165 385 567 1.0 1.3 1.15 Calcined LTX 7kaolin Example 10 18 57 116 0.057 1.61 138 364 506 1.0 1.3 1.15 CalcinedLTX 7 kaolin Example 11 18 57 116 0.057 1.61 138 364 506 1.0 1.3 1.15Alumi- LTX 6 num hydrox- ide oxide Multilayer porous membrane Thermal 2C10C Air With- shrinkage Shut- Short- 2C rate 10C rate Total permea-stand rate at down circuit rate reduc- rate reduc- thick- bility volt-150° C. temper- temper- capa- tion capa- tion ness sec/ age MD TD atureature bility rate bility rate μm 100 cc kV % % ° C. ° C. % % % % Example1 24 88 1.3 1 1 145 >200 96 0 67 3 Example 2 25 85 1.4 1 1 145 >200 96 069 4 Example 3 21 92 1.4 1 2 145 >200 96 0 73 1 Example 4 18 67 1.3 2 2145 >200 96 0 74 1 Example 5 24 150 1.4 2 2 145 >200 95 0 62 5 Example 618 72 1.0 1 2 145 >200 95 1 70 7 Example 7 16 45 1.0 1 1 144 >200 97 074 4 Example 8 24 71 1.1 1 1 145 >200 96 0 71 5 Example 9 24 87 1.3 1 1145 >200 96 0 68 1 Example 10 25 120 1.5 1 2 140 >200 97 0 65 3 Example11 24 130 1.5 1 2 140 >200 96 0 64 4

TABLE 2 Porous membrane Tortu- osity deter- Puncture mined strength Mem-from Mem- Num- in brane mem- brane Air Aver- ber terms resis- With-brane Porous layer thick- Poros- permea- age Tortu- B of of tance standresis- Layer ness ity bility pore osity pores Puncture 25 μm R volt-tance Inorganic thick- L ε sec/ size d τa pores/ strength gf/ Ω · age τbfiller Binder ness μm % 100 cc μm — μm² gf 25 μm cm² kV — — — μmComparative 26 65 75 0.060 1.18 195 358 339 1.5 1.8 1.18 Alumi- LTX 7Example 1 num hydrox- ide oxide Comparative 30 57 172 0.050 1.43 203 501415 1.9 2.0 1.16 Alumi- LTX 7 Example 2 num hydrox- ide oxideComparative 16 47 163 0.058 1.86  91 342 525 1.5 1.4 1.28 Calcined LTX 7Example 3 kaolin Comparative 16 41 157 0.085 2.10  36 366 572 2.1 1.41.41 Calcined LTX 6 Example 4 kaolin Comparative 20 41 283 0.069 2.29 50 620 760 3.2 1.8 1.58 Calcined LTX 7 Example 5 kaolin Comparative 1760 84 0.057 1.45 165 385 566 1.0 1.3 1.15 — — — Example 6 Comparative 1864 78 0.055 1.38 191 399 542 0.9 1.4 1.08 — — — Example 7 Comparative 1560 90 0.056 1.54 157 367 600 0.9 1.3 1.15 — — — Example 8 Comparative 1364 65 0.050 1.41 222 331 618 0.7 1.2 1.11 — — — Example 9 Comparative 1757 132 0.052 1.64 163 525 788 1.1 1.4 1.19 — — — Example 10 Comparative12 65 61 0.043 1.36 325 328 678 0.7 0.9 1.18 — — — Example 11Comparative 11 67 40 0.056 1.25 223 300 658 0.6 0.9 1.14 — — — Example12 Comparative 26 65 75 0.060 1.18 195 358 339 1.5 1.8 1.18 — — —Example 13 Comparative 30 57 172 0.050 1.43 203 501 415 1.9 2.0 1.16 — —— Example 14 Comparative 16 47 163 0.058 1.86  91 342 525 1.5 1.4 1.28 —— — Example 15 Comparative 16 41 157 0.085 2.10  36 366 572 2.1 1.4 1.41— — — Example 16 Comparative 20 41 283 0.069 2.29  50 619 759 3.2 1.81.58 — — — Example 17 Comparative 18 57 116 0.057 1.61 138 364 506 1  1.3 1.15 — — — Example 18 Multilayer porous membrane Thermal 2C 10C AirWith- shrinkage Shut- Short- 2C rate 10C rate Total permea- stand rateat down circuit rate reduc- rate reduc- thick- bility volt- 150° C.temper- temper- capa- tion capa- tion ness sec/ age MD TD ature aturebility rate bility rate μm 100 cc kV % % ° C. ° C. % % % % Comparative33 86 1.9 28 25 144 165 95 0 48 13 Example 1 Comparative 37 183 2.0 3027 144 162 95 0 44 12 Example 2 Comparative 23 169 1.4  1  1 145 >200 960 50 12 Example 3 Comparative 22 163 1.4  1  1 148 >200 95 0 41 15Example 4 Comparative 27 312 1.8  2  1 148 >200 94 1 34 21 Example 5Comparative 17 84 1.3 57 67 145 154 96 — 69 — Example 6 Comparative 1878 1.4 54 64 145 154 96 — 72 — Example 7 Comparative 15 90 1.3 54 59 145154 96 — 74 — Example 8 Comparative 13 65 1.2 60 67 145 154 96 — 75 —Example 9 Comparative 17 132 1.4 66 73 145 154 95 — 65 — Example 10Comparative 12 61 0.9 45 71 145 154 96 — 75 — Example 11 Comparative 1140 0.9 68 74 145 154 97 — 77 — Example 12 Comparative 26 75 1.8 67 65144 153 95 — 55 — Example 13 Comparative 30 172 2.0 72 68 144 154 95 —50 — Example 14 Comparative 16 163 1.4 73 53 145 155 96 — 57 — Example15 Comparative 16 157 1.4 60 50 148 152 95 — 48 — Example 16 Comparative20 283 1.8 63 47 148 154 95 — 43 — Example 17 Comparative 18 116 1.5 5566 140 154 96 — 69 — Example 18 *LTX . . . Arylic latex

Comparative Examples 6 to 12 and 18 use the porous membranes having anaverage pore size d=0.035 to 0.060 μm, a tortuosity τ_(a)=1.1 to 1.7,and the number B of pores=100 to 500 pores/μm², which are determined bythe gas-liquid method, and having a membrane thickness of not more than22 μm. The porous membranes in these Comparative Examples exhibit a verysmall membrane resistance (less than 1.2 Ω·cm²) and a very high value ofthe rate capability at 10 C discharge (not less than 65%). Compared tothe membrane resistances and the rate capability of the porous membranesin Comparative Examples 13 and 14 in which the membrane thickness ismore than 22 μm and those in Comparative Example 15 to 17 in whichτ_(a)>1.8 and B<100, it is clear that the porous membranes inComparative Examples 6 to 12 and 18 have very high ion conductivity. Theporous membranes in Comparative Examples 6 to 12 and 18 exhibit a highvalue of the withstand voltage (not less than 0.9 kV). It is consideredthat a large number of very small pores gather to form a finecommunication pore to attain a pore structure in which ion conductivityis high while air discharge in the membrane thickness direction barelyoccurs. The porous membranes in Comparative Examples 6 to 14 and 18 havea tortuosity determined from the membrane resistance (τb) of less than1.2. On the other hand, the thermal shrinkage rate in TD substantiallyexceeds 30% and the short-circuit temperature is approximately 154° C.in the porous membranes in Comparative Examples 6, and the heatresistance thereof is found to be insufficient.

In Examples 1 to 11, the multilayer porous membranes are prepared byforming a porous layer containing an inorganic filler and a resin binderon the porous membranes in Comparative Examples 6 to 12 and 18,respectively. Formation of the porous layer reduces the thermalshrinkage rate to a very small value (not more than 2% even at 150° C.),and improves the short-circuit temperature (more than 200° C.) andsignificantly improves the heat resistance properties. In addition, thewithstand voltage is improved (not less than 1.0 kV). The ratecapability at 10 C discharge is not less than 60% and the rate reductionrates are small (less than 10%). It concludes that the multilayer porousmembranes maintain high ionic permeability of the base material, thatis, the porous membranes in Comparative Examples 6 to 12 and 18.

In Comparative Examples 1 and 2, the multilayer porous membranes areprepared by forming a porous layer containing an inorganic filler and aresin binder on the porous membranes in Comparative Examples 13 and 14,respectively. Formation of the porous layer improves the heat resistanceproperties; however, the value of the heat resistance properties islarge (not less than 10%). The short-circuit temperature is alsoimproved, but up to approximately 165° C. In addition, the ratecapability at 10 C discharge is not more than 50%. It is clear that thisvalue is 10% or more reduced compared to those of the multilayer porousmembranes in Examples 1 to 7. Even if the porous membrane, like theporous membranes in Comparative Examples 1 and 2 as the base material,has a pore structure that satisfies an average pore size d=0.035 to0.060 μm, a tortuosity τ_(a)=1.1 to 1.7, and the number B of pores=100to 500 pores/μm², which are determined by the gas-liquid method, if ithas a large membrane thickness, it is clear that ion conductivityreduces and the degree of an improvement in the heat resistance due todisposition of the porous layer is small.

In Comparative Examples 3 to 5, the multilayer porous membranes areprepared by forming a porous layer containing an inorganic filler and aresin binder on the porous membranes in Comparative Examples 15 to 17,respectively. Formation of the porous layer significantly improves theheat resistance properties. The 2 C rate capability is approximately95%, equivalent to those in Example 1 to 11. In contrast, the ratecapability at 10 C discharge is not more than 50%, which is 10% or morereduced compared to those of the multilayer porous membrane in Examples1 to 11. The 10 C rate reduction rates are not less than 10% inComparative Examples 3 to 5, indicating that output during high outputis significantly reduced.

Examples 12 to 15 below correspond to the examples in Embodiment 2.

The physical properties in Examples were determined by the followingmethods, and other physical properties were determined by the samemethods as those in Examples above.

(15) Shrinkage Stress at 85° C.

The shrinkage stress at 85° C. was measured with a TMA50 (trademark)made by SHIMADZU Corporation. When the value in MD (TD) was measured, acut sample having a width of 3 mm in TD (MD) was prepared. The samplewas fixed to chucks such that the distance between chucks was 10 mm, andwas set in the dedicated probe. The sample was then heated from 30° C.to 200° C. at an initial load of 1.0 g and a temperature raising rate of10° C./min. The load (gf) generated during heating was measured. Theload (gf) at 85° C. was read, and the value was defined as the shrinkagestress at 85° C.

(16) Thermal Shrinkage Rate (%) at 100° C. and 150° C.

A separator was cut to prepare samples of 100 mm in MD and 100 mm in TD.The samples were left in an oven at 100° C. and an oven at 150° C. forone hour, respectively. At this time, each of the samples was sandwichedbetween two papers to prevent hot air from directly contacting thesample. After the samples were taken out from the ovens and cooled, thelengths (mm) were measured. The MD and TD thermal shrinkage rates werecalculated from the following equations, respectively:

MD thermal shrinkage rate(%)=(100−length in MD after heating)/100×100

TD thermal shrinkage rate(%)=(100−length in TD after heating)/100×100

(17) Rate Capability of Porous Membrane and Multilayer Porous Membrane

a. Production of Positive Electrode

91.2 Parts by mass of lithium nickel manganese cobalt complex oxide(Li[Ni_(1/3)Mn_(1/3)Co_(1/3)]O₂) as a positive electrode activesubstance, 2.3 parts by mass of flake graphite and 2.3 parts by mass ofacetylene black as electric conductive materials, and 4.2 parts by massof polyvinylidene fluoride (PVDF) as a resin binder were provided. Thesewere dispersed in N-methyl pyrrolidone (NMP) to prepare a slurry. Theslurry was applied onto one surface of an aluminum foil having athickness of 20 μm and serving as a positive electrode collector with adie coater such that the amount of the positive electrode activesubstance applied was 120 g/m². The coating was dried at 130° C. for 3minutes. The product was then compression-formed with a roll press suchthat the bulk density of the positive electrode active substance was2.90 g/cm³. Thus, a positive electrode was prepared. The positiveelectrode was punched into a circle having an area of 2.00 cm².

b. Production of Negative Electrode

96.6 Parts by mass of artificial graphite as a negative electrode activesubstance, and 1.4 parts by mass of ammonium salt of carboxymethylcellulose and 1.7 parts by mass of a styrene-butadiene copolymer latexas resin binders were provided. These were dispersed in purified waterto prepare a slurry. The slurry was applied onto one surface of a copperfoil having a thickness of 16 μm and serving a negative electrodecollector with a die coater such that the amount of the negativeelectrode active substance applied was 53 g/m². The coating was dried at120° C. for 3 minutes. The product was then compression-formed with aroll press such that the bulk density of the negative electrode activesubstance was 1.35 g/cm³. Thus, a negative electrode was prepared. Thenegative electrode was punched into a circle having an area of 2.05 cm².

c. Nonaqueous Electrolytic Solution

A solute LiPF₆ was dissolved at a concentration of 1.0 ml/L in a mixedsolvent of ethylene carbonate:ethyl methyl carbonate=1:2 (volume ratio)to prepare a nonaqueous electrolytic solution.

d. Assembly of Battery

The negative electrode, the multilayer porous membrane, and the positiveelectrode were laminated in this order such that the activesubstance-containing surface of the positive electrode faced that of thenegative electrode. The laminated body was placed in a stainless steelmetal container with a cover such that the copper foil of the negativeelectrode and the aluminum foil of the positive electrode contact themain body of the container and the cover, respectively. The main body ofthe container is insulated from the cover of the container. A cell wasprepared. The cell was dried under reduced pressure at 70° C. for 10hours. Subsequently, the nonaqueous electrolytic solution was injectedinto the container within an argon box, and the container was sealed toprepare a battery for evaluation.

e. Evaluation of Rate Capability

The rate capability of the separators was evaluated by preparing 10batteries for each separator by the same method as above.

The battery assembled in d. was charged to a cell voltage of 4.2 V at25° C. and a current value of 3 mA (approximately 0.5 C). Reduction ofthe current value was started from 3 mA such that 4.2 V was maintained.In such a manner, the initial charge after preparation of the batterywas performed for approximately 6 hours in total. Subsequently, thebattery was discharged at a current value of 3 mA to a cell voltage of3.0 V.

Next, the battery was charged to a cell voltage of 4.2 V at 25° C. witha current value of 6 mA (approximately 1.0 C). Reduction of the currentvalue was started from 6 mA such that 4.2 V was maintained. In such amanner, the battery was charged for approximately 3 hours in total.Subsequently, the battery was discharged at a current value of 6 mA to acell voltage of 3.0 V. The discharge capacity at this time was definedas the 1 C discharge capacity (mAh).

Next, the battery was charged to a cell voltage of 4.2 V at 25° C. witha current value of 6 mA (approximately 1.0 C). Reduction of the currentvalue was started from 6 mA such that 4.2 V was maintained. In such amanner, the battery was charged for approximately 3 hours in total.Subsequently, the battery was discharged at a current value of 60 mA(approximately 10 C) to a cell voltage of 3.0 V. The discharge capacityat this time was defined as the 10 C discharge capacity (mAh).

The proportion of the 10 C discharge capacity to the 1 C dischargecapacity was calculated, and the value was defined as the ratecapability.

10 C rate capability(%)=(10 C discharge capacity/1C dischargecapacity)×100

Using the 10 batteries prepared for each separator, the 10 C ratecapability was measured. The uniformity of output properties wasevaluated on the basis of the difference (R) between the maximum value(max) and the minimum value (min) among the values of the 10 C ratecapability obtained in the measurement.

Example 12

47.5 parts by mass of a homopolymer polyethylene having an Mv of700,000, 47.5 parts by mass of a homopolymer polyethylene having an Mvof 250,000, and 5 parts by mass of a homopolymer polypropylene having anMv of 400,000 were dry blended using a tumbler blender. 1 part by massofpentaerythrityl-tetrakis-[3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate]as an antioxidant was added to 99 parts by mass of the obtained polymermixture, and dry blended again using the tumbler blender. Thereby, amixture of the polymers and the like was obtained. After replacement bynitrogen, the obtained mixture of the polymers and the like was fed to atwin screw extruder by a feeder under a nitrogen atmosphere. Liquidparaffin (kinematic viscosity at 37.78° C.: 7.59×10⁻⁵ m²/s) was injectedinto a cylinder of the extruder by a plunger pump.

The feeder and the pump were adjusted such that the ratio of the amountof liquid paraffin to the mixture of all the components melt kneaded andextruded was 67% by mass (resin composition content: 33% by mass). Forthe melt kneading conditions, a preset temperature was 200° C., a screwrotation speed was 100 rpm, and an amount of discharge was 12 kg/h.

Then, the melt kneaded product was extruded through a T die and castonto a cooling roll, whose surface temperature was controlled to be 25°C., to obtain a gel sheet having a thickness of 1600 μm.

Next, the gel sheet was guided to a simultaneous biaxial tenterstretching machine to be subjected to biaxial stretching. For thestretching condition settings, the MD magnification was 7.0 times, theTD magnification was 6.1 times, and a preset temperature was 121° C.

Next, the sheet was guided to a tank of methyl ethyl ketone, and wassufficiently immersed in methyl ethyl ketone to remove liquid paraffinby extraction. Subsequently methyl ethyl ketone was removed by drying.

Next, the sheet was guided to a TD tenter, and subjected to heatsetting. The heat setting temperature was 120° C., the largest TDmagnification was 2.0 times, and the relaxation rate was 0.90. Then, thesheet was relaxed in MD with a speed-variable heating roll. At thistime, the relaxation rate was 0.95. As a result, a polyolefin resinporous membrane having: a membrane thickness of 17 μm; a porosity of60%; an air permeability of 88 seconds/100 cc; an average pore sized=0.057 μm, a tortuosity τa=1.45, and the number B of pores=165pores/μm², which were determined by the gas-liquid method; and apuncture strength of 567 gf in terms of 25 μm, was obtained.

Next, 95.0 parts by mass of calcined kaolin prepared by calcining wetkaolin(kaolinite (Al₂Si₂O₅(OH)₄) as a main component) at a hightemperature (average particle size: 1.8 μm), 5.0 parts by mass of anacrylic latex (solid content: 40%, average particle size: 220 nm, lowestmembrane forming temperature: 0° C. or less), and 0.5 parts by mass ofan aqueous solution of ammonium polycarboxylate (SN-DISPERSANT 5468 madeby SAN NOPCO Limited) were uniformly dispersed in 180 parts by mass ofwater to prepare a coating solution. The coating solution was appliedonto the surface of the polyolefin resin porous membrane with a gravurecoater. Water was removed by drying at 60° C. to obtain a multilayerporous membrane including the polyolefin resin porous membrane and aporous layer having a thickness of 7 μm and formed on the polyolefinresin porous membrane.

Example 13

A polyolefin resin porous membrane having: a membrane thickness of 13μm; a porosity of 64%; an air permeability of 67 seconds/100 cc; anaverage pore size d=0.050 μm, a tortuosity τa=1.41, and the number B ofpores=222 pores/μm², which were determined by the gas-liquid method; anda puncture strength of 618 gf in terms of 25 μm, was obtained in thesame manner as that in Example 12 except that the melt kneaded productwas cast through the T die to prepare a gel sheet having a thickness of1150 μm, a preset temperature for biaxial stretching with thesimultaneous biaxial tenter stretching machine was 120° C., and therelaxation rate in MD was 0.92.

Next, 96.0 Parts by mass of aluminum hydroxide oxide (average particlesize: 1.0 μm), 4.0 parts by mass of an acrylic latex (solid content:40%, average particle size: 140 nm, lowest membrane forming temperature:0° C. or less), and 0.5 parts by mass of an aqueous solution of ammoniumpolycarboxylate (SN-DISPERSANT 5468 made by SAN NOPCO Limited) wereuniformly dispersed in 180 parts by mass of water to prepare a coatingsolution. The coating solution was applied onto the surface of thepolyolefin resin porous membrane with a gravure coater. Water wasremoved by drying at 60° C. to obtain a multilayer porous membraneincluding a polyolefin resin porous membrane and a porous layer having athickness of 7 μm and formed on the polyolefin resin porous membrane.

Example 14

A multilayer porous membrane was obtained by the same method as that inExample 13 except that the porous layer containing aluminum hydroxideoxide and formed on the polyolefin resin porous membrane had a thicknessof 4 μm.

Comparative Example 19

A multilayer porous membrane was obtained by the same method as that inExample 12 except that thermal relaxation in MD was not performed.

Comparative Example 20

A multilayer porous membrane was obtained by the same method as that inExample 12 except that the heat setting temperature in the TD tenter was132° C., and thermal relaxation in MD was not performed.

Comparative Example 21

A multilayer porous membrane was obtained by the same method as that inExample 13 except that thermal relaxation in MD was not performed, andthe porous layer containing aluminum hydroxide oxide had a thickness of2 μm.

Comparative Example 22

47.5 parts by mass of a homopolymer polyethylene having an My of700,000, 47.5 parts by mass of a homopolymer polyethylene having an Mvof 250,000, and 5 parts by mass of a homopolymer polypropylene having anMv of 400,000 were dry blended using a tumbler blender. 1 part by massofpentaerythrityl-tetrakis-[3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate]as an antioxidant was added to 99 parts by mass of the obtained polymermixture, and dry blended again using the tumbler blender. Thereby, amixture of the polymers and the like was obtained. After replacement bynitrogen, the obtained mixture of the polymers and the like was fed to atwin screw extruder by a feeder under a nitrogen atmosphere. Liquidparaffin (kinematic viscosity of 7.59×10⁻⁵ m²/s at 37.78° C.) as aplasticizer was injected into a cylinder of the extruder by a plungerpump. The feeder and the pump were adjusted such that the ratio of theamount of the liquid paraffin to the mixture of all the components meltkneaded and extruded was 65% by mass. For the melt kneading conditions,a preset temperature was 200° C., a screw rotation speed was 240 rpm,and an amount of discharge was 12 kg/h.

Then, the melt kneaded product was extruded through a T die and castonto a cooling roll, whose surface temperature was controlled to be 25°C., to obtain a sheet-like polyolefin composition having a thickness of1300 μm.

Next, the polyolefin composition was guided to a simultaneous biaxialtenter stretching machine, and subjected to simultaneous biaxialstretching of 7 times in the MD and 6.4 times in the TD. At this time,the preset temperature of the simultaneous biaxial tenter was 118° C.Next, the stretched polyolefin composition was guided to a methyl ethylketone tank, and the liquid paraffin was removed by extraction.Subsequently, methyl ethyl ketone was removed by drying.

Next, the polyolefin composition was guided to a TD tenter, andsubjected to heat setting. The heat setting temperature was 122° C., thelargest TD magnification was 1.4 times, and the relaxation rate was0.85. As a result, a polyolefin resin porous membrane having: a membranethickness of 16 μm; a porosity of 47%; an air permeability of 163seconds/100 cc; an average pore size d=0.058 μm, a tortuosity τa=1.86,and the number of pores B=91 pores/μm², which were determined by thegas-liquid method; and a puncture strength of 525 gf in terms of 25 μm,was obtained.

Next, 95.0 parts by mass of calcined kaolin prepared by calcining wetkaolin (kaolinite (Al₂Si₂O₅(OH)₄) as a main component) at a hightemperature (average particle size: 1.8 μm), 5.0 parts by mass of anacrylic latex (solid content: 40%, average particle size: 220 nm, lowestmembrane forming temperature: 0° C. or less), and 0.5 parts by mass ofan aqueous solution of ammonium polycarboxylate (SN-DISPERSANT 5468 madeby SAN NOPCO Limited) were uniformly dispersed in 180 parts by mass ofwater to prepare a coating solution. The coating solution was applied tothe surface of the polyolefin resin porous membrane using a microgravure coater. Water was removed by drying at 60° C. to obtain amultilayer porous membrane including the polyolefin resin porousmembrane and a porous layer having a thickness of 7 μm and formed on thepolyolefin resin porous membrane.

Example 15

A porous membrane was obtained in the same manner as that in Example 12except that no inorganic filler porous layer was formed.

Comparative Example 23

A porous membrane was obtained in the same manner as that in ComparativeExample 19 except that no inorganic filler porous layer was formed.

The physical properties and the results of evaluation of the porousmembranes, the porous layers, and the multilayer porous membranesobtained in Examples and Comparative Examples are shown in Table 3below.

TABLE 3 Comparative Comparative Unit Example 12 Example 13 Example 14Example 19 Example 20 Porous Membrane thickness μm 17 13 13 17 12membrane Porosity % 60 64 64 60 41 Air permeability sec./100 cc 88 67 6784 92 Shrinkage stress at gf 1.8 1.6 1.6 2.3 1.7 85° C. Average poresize μm 0.057 0.050 0.050 0.057 0.085 Tortuosity — 1.45 1.41 1.41 1.452.1 Number of pores pores/μm² 165 222 222 165 36 Puncture strength gf385 331 331 385 288 Porous Inorganic filler — Calcined Aluminum AluminumCalcined Calcined layer kaolin hydroxide hydroxide kaolin kaolin oxideoxide Binder — Acrylic latex Acrylic latex Acrylic latex Acrylic latexAcrylic latex Layer thickness μm 7 7 4 7 7 Multilayer Total thickness μm24 20 17 24 19 porous Air permeability sec./100 cc 92 77 75 88 96membrane Thermal MD % <1 <1 <1 <1 <1 shrinkage TD % <1 <1 <1 <1 <1 (100°C.) Thermal MD % 1 1 2 1 1 shrinkage TD % 1 1 2 1 1 (150° C.) Rate ave.% 67 70 70 67 51 capability max. % 70.4 72.3 73.2 70.6 53.4 min. % 65.768.8 66.8 60.5 48.5 R % 4.7 3.5 6.4 10.1 4.9 Comparative ComparativeComparative Unit Example 21 Example 22 Example 15 Example 23 PorousMembrane thickness μm 13 16 17 17 membrane Porosity % 64 47 60 60 Airpermeability sec./100 cc 65 163 88 84 Shrinkage stress at gf 2.5 3.2 1.82.3 85° C. Average pore size μm 0.050 0.058 0.057 0.057 Tortuosity —1.41 1.86 1.45 1.45 Number of pores pores/μm² 222 91 165 165 Puncturestrength gf 331 342 385 385 Porous Inorganic filler — Aluminum Calcined— — layer hydroxide kaolin oxide Binder — Acrylic latex Acrylic latex —— Layer thickness μm 2 7 — — Multilayer Total thickness μm 15 23 17 17porous Air permeability sec./100 cc 76 169 88 84 membrane Thermal MD % 6<1 8 11 shrinkage TD % 4 0 6 8 (100° C.) Thermal MD % 48 1 54 55shrinkage TD % 52 1 67 68 (150° C.) Rate ave. % 68 50 65 66 capabilitymax. % 72.1 53.8 69.9 70.1 min. % 61.6 42.8 60.2 57.2 R % 10.5 11 9.712.9

Examples 12 to 14 and Comparative Examples 19 and 21 exhibit a very highvalue of the rate capability at 10 C discharge (not less than 65%).Comparative Examples 19 and 21, however, have a fluctuation (max−min) ofapproximately 10% of the rate capability at 10 C discharge. In contrast,Examples 12 to 14 have a small fluctuation (max−min), which is 3.5 to6.4%. This is probably because the shrinkage stress at 85° C. isadjusted to fall within the small range (not more than 2.2 gf).

From comparison between Example 15 and Comparative Example 23, Example15 has a small fluctuation (max−min) in the rate capability. This isprobably because the shrinkage stress at 85° C. is adjusted to fallwithin the small range (not more than 2.2 gf).

Comparison of Examples 13 and 14 to Comparative Examples 22 and Example15 indicates that the porous layers in Examples 13 and 14 have a layerthickness of not less than 3 μm and exhibit not only excellent thermalshrinkage property but also high rate capability.

Comparative Example 21 exhibits inferior rate capability due to its lowporosity.

Examples 16 to 24 below correspond to the examples of Embodiment 3.

The physical properties in Examples were determined by the followingmethods, and other physical properties were determined by the samemethod as that in Examples above.

(18) Shutdown Rate

Two nickel foils (A and B) having a thickness of 10 μm were prepared.With a Teflon tape, Nickel foil A was fixed on a glass plate and maskedsuch that a 10 mm square portion of the foil was left.

Another Nickel foil B was disposed on a ceramic plate connected to athermocouple. A measurement sample of a microporous membrane immersed ina prescribed electrolytic solution for 3 hours was disposed on Nickelfoil B. The glass plate having Nickel foil A attached was disposed onthe sample, and silicon rubber was further disposed thereon.

The laminate was set on a hot plate, and the temperature was then raisedat a rate of 2° C./min or 18° C./min while a pressure of 1.5 MPa wasapplied with a hydraulic press.

At this time, changes in impedance were measured at an alternatingcurrent of 1 V and 1 kHz. In the measurement, the temperature when theimpedance reached 1000Ω was defined as a fuse temperature. Thetemperature when pores were clogged and then the impedance reached lessthan 1000Ω again was defined as the short-circuit temperature. The timeneeded to increase the impedance from 100Ω to 1000Ω during an increasein the temperature was defined as a shutdown rate (R).

R(seconds)=(t(1000)−t(100))/V(t)×60

t(100): temperature when the impedance reaches 100 Ω

t(1000): temperature when the impedance reaches 1000 Ω

V(t): temperature raising rate (2° C./min or 18° C./min)

The prescribed electrolytic solution has the following compositionratio.

composition ratio of solvents (volume ratio): propylenecarbonate/ethylene carbonate/γ-butyllactone=1/1/2

composition ratio of the electrolytic solution: LiBF₄ is dissolved at aconcentration of 1 mol/L in the solvent above, and trioctyl phosphatewas further added such that the concentration is 0.5% by weight.

(19) Nail Penetration Evaluation <Production of Positive Electrode>

A mixed positive electrode active substance: 85 parts by mass (preparedby mechanically mixing lithium nickel manganese cobalt complex oxidepowder (LiNi_(1/3)Mn_(1/3)Co_(1/3)O₂) with lithium manganese complexoxide powder (LiMn₂O₄) as positive electrode active substances in a massratio of 70:30), acetylene black as a conductive aid: 6 parts by mass,and PVDF as a binder: 9 parts by mass were uniformly mixed inN-methyl-2-pyrrolidone (NMP) as a solvent to prepare a paste containinga positive electrode mixture. The paste containing the positiveelectrode mixture was uniformly applied onto both surfaces of acollector composed of an aluminum foil and having a thickness of 20 μm,and was dried. The product was then compression-formed with a roll pressto adjust the thickness of the layer of the positive electrode mixturesuch that the product had a total thickness of 130 μm. A positiveelectrode of a rectangular sheet (short side: 95 mm, long side: 120 mm)having a lead tab terminal disposed on the upper short side thereof wasprepared, wherein the lead tab terminal was composed of an aluminum foilhaving a length of 20 mm and applied no active substance.

<Production of Negative Electrode>

Graphite as a negative electrode active substance: 91 parts by mass andPVDF as a binder: 9 parts by mass were uniformly mixed in NMP as asolvent to prepare a paste containing a negative electrode mixture. Thepaste containing a negative electrode mixture was uniformly applied ontoboth surfaces of a collector composed of a copper foil and having athickness of 15 μm, and was dried. The product was thencompression-formed with a roll press to adjust the thickness of thelayer of the negative electrode mixture such that the product had atotal thickness of 130 μm. A negative electrode of a rectangular sheet(short side: 95 mm, long side: 120 mm) having a lead tab terminaldisposed on the upper short side thereof was prepared, wherein the leadtab terminal was composed of a copper foil having a length of 20 mm andapplied no active substance.

<Preparation of Nonaqueous Electrolytic Solution>

A solute LiPF₆ was dissolved at a concentration of 1.0 mol/L in a mixedsolvent of ethylene carbonate:ethyl methyl carbonate:dimethylcarbonate=1:1:1 (volume ratio) to prepare a nonaqueous electrolyticsolution.

<Production of Cell>

27 positive electrode sheets above and 28 negative electrode sheetsabove were alternatingly laminated while a separator was interposedbetween the positive electrode sheet and the negative electrode sheet toseparate the positive electrode sheet from the negative electrode sheet.Thus, a laminate body of electrode plates was prepared. The separatorwas a strip-like separator having a width of 125 mm, and was folded in azigzag pattern (99 times folded) to prepare the laminate body ofelectrode plates. A schematic view of the laminate body of electrodeplates is shown in FIG. 5.

The laminate body of electrode plates was pressed into a flat plate. Theflat plate was then accommodated in an aluminum laminate film, and threesides of the film were heat sealed. The lead tab terminal of thepositive electrode and that of the negative electrode were projectedfrom one side of the laminate film. After drying, the nonaqueouselectrolytic solution was injected into the container, and the unsealedside was sealed. The lithium ions battery thus prepared was designed tohave a capacity of 10 Ah.

<Nail Penetration Evaluation>

The laminate cell was constant-current constant-voltage (CCCV) chargedfor 3 hours at a current value of 3 A (0.3 C) and a final cell voltageof 4.2 V. The laminate cell was settled on an iron plate within anexplosion-proof booth. An iron nail having a diameter of 2.5 mm waspenetrated through the center of the cell under an environment around25° C. at a rate of 3 mm/second. The nail was penetrated through thecell and maintained. The cell was determined as unacceptable (X) if thecell ignited or exploded in 15 minutes, and as acceptable (◯) if thecell did not ignite or explode in 15 minutes.

Example 16

47.5 Parts by mass of homopolymer polyethylene having an My of 700000,47.5 parts by mass of homopolymer polyethylene having an Mv of 250000,and 5 parts by mass of homopolymer polypropylene having an Mv of 400000were dry blended with a tumbler blender. As an antioxidant, 1 wt %pentaerythrityl-tetrakis-[3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate]was added to 99 wt % pure polymer, and was dry blended with a tumblerblender again to prepare a mixture of the polymers and the like. Afterreplacement by nitrogen, the obtained mixture of the polymers and thelike was fed to a twin screw extruder under a nitrogen atmosphere with afeeder. Liquid paraffin (kinematic viscosity at 37.78° C.: 7.59×10⁻⁵m²/s) was injected into an extruder cylinder with a plunger pump.

The feeder and the pump were adjusted such that the ratio of the amountof liquid paraffin to the mixture of all the components melt kneaded andextruded was 66 wt % (resin composition content: 34%). For the meltkneading conditions, a preset temperature was 200° C., a screw rotationspeed was 100 rpm, and an amount of discharge was 12 kg/h.

Then, the melt kneaded product was extruded through a T die and castonto a cooling roll, whose surface temperature was controlled to be 25°C., to obtain a gel sheet having a thickness of 1570 μm.

Next, the gel sheet was guided to a simultaneous biaxial tenterstretching machine to be subjected to biaxial stretching. For thestretching condition settings, the MD magnification was 7.0 times, theTD magnification was 6.1 times, and a preset temperature was 119° C.

Next, the sheet was guided to a tank of methyl ethyl ketone, and wassufficiently immersed in methyl ethyl ketone to remove liquid paraffinby extraction. Subsequently methyl ethyl ketone was removed by drying.

Next, the sheet was guided to a TD tenter, and subjected to heatsetting. The heat setting temperature was 127° C., the largest TDmagnification was 2.0 times, and the relaxation rate was 0.9. Thephysical properties of the polyolefin resin porous membrane obtained areshown in Table 4.

Next, 92.0 Parts by mass of aluminum hydroxide oxide (average particlesize: 1.0 μm), 8.0 parts by mass of an acrylic latex suspension (solidcontent: 40%, average particle size: 150 nm), and 1.0 part by mass of anaqueous solution of ammonium polycarboxylate (SN-DISPERSANT 5468 made bySAN NOPCO Limited) were uniformly dispersed in 100 parts by mass ofwater to prepare a coating solution. The coating solution was appliedonto the surface of the polyolefin resin porous membrane with a microgravure coater. Water was removed by drying at 60° C. to obtain amultilayer porous membrane including a polyolefin resin porous membraneand a porous layer having a thickness of 7 μm and formed on thepolyolefin resin porous membrane. The physical properties of themultilayer porous layer obtained are shown in Table 4.

Example 17

A coating layer was formed with calcined kaolin prepared by calciningwet kaolin(kaolinite (Al₂Si₂O₅(OH)₄) as a main component) at a hightemperature, average particle size: 1.8 μm) instead of aluminumhydroxide oxide on the surface of the polyolefin resin porous membraneobtained in Example 16. 90.0 Parts by mass of calcined kaolin, 10.0parts by mass of an acrylic latex suspension (solid content: 40%,average particle size: 150 nm), and 0.5 parts by weight of an aqueoussolution of ammonium polycarboxylate (SN-DISPERSANT 5468 made by SANNOPCO Limited) were uniformly dispersed in 180 parts by mass of water toprepare a coating solution. The coating solution was applied onto thesurface of the polyolefin resin porous membrane prepared in Example 14with a gravure coater. Water was removed by drying at 60° C. to obtain amultilayer porous membrane including a polyolefin resin porous membraneand a porous layer formed on the polyolefin resin porous membrane andhaving a thickness of 7 μm. The physical properties of the polyolefinresin porous membrane and the multilayer porous layer obtained are shownin Table 4.

Example 18

A multilayer porous membrane was obtained in the same manner as that inExample 16 except that the heat setting temperature was 121° C. Thephysical properties of the polyolefin resin porous membrane and themultilayer porous layer obtained are shown in Table 4.

Example 19

A multilayer porous membrane was obtained in the same manner as that inExample 18 except that the binder was replaced with an acrylic latexsuspension (solid content: 40%, average particle size: 60 nm) having anaverage particle size of 60 nm. The physical properties of thepolyolefin resin porous membrane and the multilayer porous layerobtained are shown in Table 4.

Example 20

A multilayer porous membrane was obtained in the same manner as that inExample 18 except that the binder was replaced with an acrylic latexsuspension (solid content: 40%, average particle size: 460 nm) having anaverage particle size of 460 nm. The physical properties of thepolyolefin resin porous membrane and the multilayer porous layerobtained are shown in Table 4.

Example 21

A multilayer porous membrane was obtained in the same manner as that inExample 16 except that heat setting with the TD tenter was notperformed. The physical properties of the polyolefin resin porousmembrane and the multilayer porous layer obtained are shown in Table 4.

Example 22

95 Parts by mass of homopolymer polyethylene having an Mv of 250000, and5 parts by mass of homopolymer polypropylene having an Mv of 400000 weredry blended with a tumbler blender. As an antioxidant, 1 wt %pentaerythrityl-tetrakis-[3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate]was added to 99 wt % pure polymer, and was dry blended with a tumblerblender again to prepare a mixture of the polymers and the like. Afterreplacement by nitrogen, the obtained mixture of the polymers and thelike was fed to a twin screw extruder under a nitrogen atmosphere with afeeder. Liquid paraffin (kinematic viscosity at 37.78° C.: 7.59×10⁻⁵m²/s) was injected into an extruder cylinder with a plunger pump.

The feeder and the pump were adjusted such that the ratio of the amountof liquid paraffin to the mixture of all the components melt kneaded andextruded was 62 wt % (PC content: 38%). For the melt kneadingconditions, a preset temperature was 200° C., a screw rotation speed was100 rpm, and an amount of discharge was 12 kg/h.

Then, the melt kneaded product was extruded through a T die and castonto a cooling roll, whose surface temperature was controlled to be 25°C., to obtain a gel sheet having a thickness of 1250 μm.

Next, the gel sheet was guided to a simultaneous biaxial tenterstretching machine to be subjected to biaxial stretching. For thestretching condition settings, the MD magnification was 7.0 times, theTD magnification was 5.2 times, and a preset temperature was 123° C.

Next, the sheet was guided to a tank of methyl ethyl ketone, and wassufficiently immersed in methyl ethyl ketone to remove liquid paraffinby extraction. Subsequently methyl ethyl ketone was removed by drying.

Next, the sheet was guided to a TD tenter, and subjected to heatsetting. The heat setting temperature was 125° C., the largest TDmagnification was 3.5 times, and the relaxation rate was 0.94. Thephysical properties of the polyolefin resin porous membrane obtained areshown in Table 4.

Next, 92.0 parts by mass of aluminum hydroxide oxide (average particlesize: 1.0 μm), 8.0 parts by mass of an acrylic latex suspension (solidcontent: 40%, average particle size: 150 nm), and 1.0 part by mass of anaqueous solution of ammonium polycarboxylate (SN-DISPERSANT 5468 made bySAN NOPCO Limited) were uniformly dispersed in 100 parts by mass ofwater to prepare a coating solution. The coating solution was appliedonto the surface of the polyolefin resin porous membrane with a microgravure coater. Water was removed by drying at 60° C. to obtain amultilayer porous membrane including a polyolefin resin porous membraneand a porous layer having a thickness of 6 μm and disposed on thepolyolefin resin porous membrane. The physical properties of themultilayer porous layer obtained are shown in Table 4.

Example 23

26 Parts by mass of homopolymer polyethylene having an Mv of 250000, 16parts by mass of homopolymer polyethylene having an Mv of 2000000, 8parts by mass of silica “DM10C” (trademark, made by TokuyamaCorporation, hydrophobized with dimethyldichlorosilane) having anaverage primary particle size of 15 nm, 10 parts by mass of liquidparaffin “Smoil P-350P” (trademark, made by MATSUMURA OIL RESEARCHCORP.) as a plasticizer, and 0.3 parts by mass ofpentaerythrityl-tetrakis-[3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate]as an antioxidant were added, and were premixed with a SUPERMIXER. Theobtained premixture was fed with a feeder to a feed port of a biaxialunidirectional screw type extruder. The liquid paraffin was side fed toa twin screw extruder cylinder such that the ratio of the liquidparaffin was 65 parts by mass to the whole mixture (100 parts by mass)melt kneaded and extruded. For the melt kneading conditions, a presettemperature was 200° C., a screw rotation speed was 180 rpm, and anamount of discharge was 12 kg/h. Then, the melt kneaded product wasextruded through a T die onto a cooling roll, whose surface temperaturewas controlled to be 25° C., thereby to obtain a sheet-like polyolefincomposition having a thickness of 1100 μm. Next, the sheet was guided toa simultaneous biaxial tenter stretching machine, and was subjected tobiaxial stretching. For the stretching condition settings, the MDmagnification was 7.0 times, the TD magnification was 6.2 times, and apreset temperature was 122° C. Next, the sheet was guided to a tank ofmethylene chloride, and was sufficiently immersed in methylene chlorideto remove liquid paraffin by extraction. Subsequently methylene chloridewas dried. Next, the sheet was guided to a TD tenter, and subjected toheat setting. The heat setting temperature was 127° C., the largest TDmagnification was 2.0 times, and the relaxation rate was 0.9. Thephysical properties of the polyolefin resin porous membrane obtained areshown in Table 4.

Next, 92.0 parts by mass of aluminum hydroxide oxide (average particlesize: 1.0 μm), 8.0 parts by mass of an acrylic latex suspension (solidcontent: 40%, average particle size: 150 nm), and 1.0 part by mass of anaqueous solution of ammonium polycarboxylate (SN-DISPERSANT 5468 made bySAN NOPCO Limited) were uniformly dispersed in 100 parts by mass ofwater to prepare a coating solution. The coating solution was appliedonto the surface of the polyolefin resin porous membrane with a microgravure coater. Water was removed by drying at 60° C. to obtain amultilayer porous membrane including a polyolefin resin porous membraneand a porous layer having a thickness of 7 μm and formed on thepolyolefin resin porous membrane. The physical properties of themultilayer porous layer obtained are shown in Table 4.

Example 24

A coating layer mainly containing aluminum hydroxide oxide was formed onboth surfaces of the polyolefin resin porous membrane obtained inExample 18. 92.0 Parts by mass of aluminum hydroxide oxide (averageparticle size: 1.0 μm), 8.0 parts by mass of an acrylic latex suspension(solid content: 40%, average particle size: 150 nm), and 1.0 part byweight of an aqueous solution of ammonium polycarboxylate (SN-DISPERSANT5468 made by SAN NOPCO Limited) were uniformly dispersed in 100 parts bymass of water to prepare a coating solution. The coating solution wasapplied onto one surface of the polyolefin resin porous membraneobtained in Example 16 with a micro gravure coater. Water was removed bydrying at 60° C. to form a porous layer having a thickness of 4 μm onthe polyolefin resin porous membrane. Next, another porous layer havinga thickness of 4 μm was formed on the other surface of the polyolefinresin porous membrane to obtain a 3-layered multilayer porous membrane.The physical properties of the polyolefin resin porous membrane and themultilayer porous layer obtained are shown in Table 4.

Comparative Example 24

A coating layer was formed with polyvinyl alcohol as a binder. 3.5 Partsby mass of polyvinyl alcohol (average degree of polymerization: 1700,degree of saponification: not less than 99%) was uniformly dissolved in150 parts by mass of water; and 96.5 parts by mass of aluminum hydroxideoxide (average particle size: 1.8 μm) was then added thereto, and wasuniformly dispersed to prepare a coating solution. The coating solutionwas applied onto the surface of the polyolefin resin porous membraneobtained in Example 18 with a micro gravure coater. Water was removed bydrying at 60° C. to obtain a multilayer porous membrane including apolyolefin resin porous membrane and a porous layer formed on thepolyolefin resin porous membrane and having a thickness of 7 μm. Thephysical properties of the polyolefin resin porous membrane and themultilayer porous layer obtained are shown in Table 5.

Comparative Example 25

A coating layer was formed with a polyvinylidene fluoride copolymer as abinder. 100 Parts by mass of aluminum hydroxide oxide (average particlesize: 1.8 μm) was added to 150 parts by weight of a polyvinylidenefluoride (PVDF)-hexafluoropropylene (HFP) copolymer solution (HFP: 1 mol%, 5 wt % NMP solution,), and was uniformly dispersed to prepare acoating solution. The coating solution was applied onto the surface ofthe polyolefin resin porous membrane obtained in Example 16 with a microgravure coater. The membrane was immersed in a water bath, and NMP wasthen washed off with hot water at 60° C. The membrane was then dried toremove water to obtain a multilayer porous membrane including apolyolefin resin porous membrane and a porous layer formed on thepolyolefin resin porous membrane and having a thickness of 7 μm. Thephysical properties of the polyolefin resin porous membrane and themultilayer porous layer obtained are shown in Table 5.

Comparative Example 26

A multilayer porous membrane was obtained in the same manner as that inExample 18 except that the binder was replaced with an acrylic latexsuspension (solid content: 40%, average particle size: 600 nm) having anaverage particle size of 600 nm. The physical properties of thepolyolefin resin porous membrane and the multilayer porous layerobtained are shown in Table 5.

Comparative Example 27

A multilayer porous membrane was obtained in the same manner as that inExample 18 except that the binder was replaced with an acrylic latexsuspension (solid content: 40%, average particle size: 40 nm) having anaverage particle size of 40 nm. The physical properties of thepolyolefin resin porous membrane and the multilayer porous layerobtained are shown in Table 5.

Comparative Example 28

A multilayer porous membrane was obtained in the same manner as that inExample 16 except that the heat setting temperature was 133° C., thelargest TD magnification was 2.6 times, and the relaxation rate was0.96. The physical properties of the polyolefin resin porous membraneand the multilayer porous layer obtained are shown in Table 5.

Comparative Example 29

47.5 Parts by mass of homopolymer polyethylene having an Mv of 700000,47.5 parts by mass of homopolymer polyethylene having an Mv of 250000,and 5 parts by mass of homopolymer polypropylene having an My of 400000were dry blended with a tumbler blender. As an antioxidant, 1% by massofpentaerythrityl-tetrakis-[3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate]was added to 99% by mass of the obtained pure polymer mixture, and wasdry blended with the tumbler blender again to prepare a mixture of thepolymers and the like. After replacement by nitrogen, the obtainedmixture of the polymers and the like was fed to a twin screw extruderunder a nitrogen atmosphere with a feeder. Liquid paraffin (kinematicviscosity at 37.78° C.: 7.59×10⁻⁵ m²/s) as a plasticizer was injectedinto an extruder cylinder with a plunger pump. The feeder and the pumpwere adjusted such that the ratio of the amount of liquid paraffin tothe mixture of all the components melt kneaded and extruded was 68% bymass. For the melt kneading conditions, a preset temperature was 200°C., a screw rotation speed was 240 rpm, and an amount of discharge was12 kg/h.

Then the melt kneaded product was extruded through a T die and cast ontoa cooling roll, whose surface temperature was controlled to be 25° C.,to obtain a sheet-like polyolefin composition having a thickness of 1300μm.

Next, the sheet was guided to a simultaneous biaxial tenter stretchingmachine, and was subjected to simultaneous biaxial stretching of 7 timesin MD and 6.1 times in TD. At this time, a preset temperature in thesimultaneous biaxial tenter was 117° C. Next, the sheet was guided to atank of methyl ethyl ketone, and liquid paraffin was removed byextraction. Thereafter, methyl ethyl ketone was removed by drying. Next,the sheet was guided to a TD tenter, and subjected to heat setting. Theheat setting temperature was 122° C., the largest TD magnification was1.4 times, and the relaxation rate was 0.85. The physical properties ofthe polyolefin resin porous membrane obtained are shown in Table 5.

Next, 92.0 parts by mass of aluminum hydroxide oxide (average particlesize: 1.0 μm), 8.0 parts by mass of an acrylic latex suspension (solidcontent: 40%, average particle size 150 nm), and 1.0 part by mass of anaqueous solution of ammonium polycarboxylate (SN-DISPERSANT 5468 made bySAN NOPCO Limited) were uniformly dispersed in 100 parts by mass ofwater to prepare a coating solution. The coating solution was appliedonto the surface of the polyolefin resin porous membrane with a microgravure coater. Water was removed by drying at 60° C. to obtain amultilayer porous membrane including a polyolefin resin porous membraneand a porous layer having a thickness of 7 μm and formed on thepolyolefin resin porous membrane. The physical properties of themultilayer porous layer obtained are shown in Table 5.

Comparative Example 30

47 Parts by mass of homopolymer polyethylene having an Mv of 700000, 46parts by mass of homopolymer polyethylene having an Mv of 250000, and 7parts by mass of homopolymer polypropylene having an Mv of 400000 weredry blended with a tumbler blender. As an antioxidant, 1% by mass ofpentaerythrityl-tetrakis-[3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate]was added to 99% by mass of the obtained pure polymer mixture, and wasdry blended with the tumbler blender again to prepare a mixture of thepolymers and the like. After replacement by nitrogen, the obtainedmixture of the polymers and the like was fed to a twin screw extruderunder a nitrogen atmosphere with a feeder. Liquid paraffin (kinematicviscosity at 37.78° C.: 7.59×10⁻⁵ m²/s) as a plasticizer was injectedinto an extruder cylinder with a plunger pump. The feeder and the pumpwere adjusted such that the ratio of the amount of liquid paraffin tothe mixture of all the components melt kneaded and extruded was 65% bymass. For the melt kneading conditions, a preset temperature was 200°C., a screw rotation speed was 240 rpm, and an amount of discharge was12 kg/h.

Then the melt kneaded product was extruded through a T die and cast ontoa cooling roll, whose surface temperature was controlled to be 25° C.,thereby to obtain a sheet-like polyolefin composition having a thicknessof 1700 μm.

Next, the sheet was guided to a simultaneous biaxial tenter stretchingmachine, and was subjected to simultaneous biaxial stretching of 7 timesin MD and 6.1 times in TD. At this time, a preset temperature in thesimultaneous biaxial tenter was 125° C. Next, the sheet was guided to atank of methyl ethyl ketone, and liquid paraffin was removed byextraction. Thereafter, methyl ethyl ketone was removed by drying. Next,the sheet was guided to a TD tenter, and subjected to heat setting. Theheat setting temperature was 125° C., the largest TD magnification was1.7 times, and the relaxation rate was 0.82. The physical properties ofthe polyolefin resin porous membrane obtained are shown in Table 5.

Next, 92.0 parts by mass of aluminum hydroxide oxide (average particlesize: 1.0 μm), 8.0 parts by mass of an acrylic latex suspension (solidcontent: 40%, average particle size: 150 nm), and 1.0 part by mass of anaqueous solution of ammonium polycarboxylate (SN-DISPERSANT 5468 made bySAN NOPCO Limited) were uniformly dispersed in 100 parts by mass ofwater to prepare a coating solution. The coating solution was appliedonto the surface of the polyolefin resin porous membrane with a microgravure coater. Water was removed by drying at 60° C. to obtain amultilayer porous membrane including a polyolefin resin porous membraneand a porous layer having a thickness of 7 μm and formed on thepolyolefin resin porous membrane. The physical properties of themultilayer porous layer obtained are shown in Table 5.

FIG. 1 shows a graph on shutdown measurement at a temperature raisingrate of 2° C./min in Example 16 and Comparative Example 24 while FIG. 2shows a graph on shutdown measurement at a temperature raising rate of18° C./min in Example 16 and Comparative Example 24. When thetemperature raising rate is 2° C./min, the shutdown rate in Example 16is similar to that in Comparative Example 24 (Example 14: 24 seconds,Comparative Example 22: 24 seconds). When the temperature raising rateis 18° C./min, the shutdown rates are different (Example 14: 8.7seconds, Comparative Example 22: 21 seconds). The graphs indicate thatthe polyolefin resin porous membrane in Example 16 exhibits a shutdownbehavior in a short time at a high temperature raising rate.

FIG. 3 shows a graph on the results of nail penetration evaluation inExample 16 while FIG. 4 shows a graph on the results of nail penetrationevaluation in Comparative Example 24. The polyolefin resin porousmembrane in Example 16 did not ignite while the polyolefin resin porousmembrane in Comparative Example 24 ignited and exploded. The outersurface temperature of the cell including the polyolefin resin porousmembrane in Example 16 was around 60° C. After the nail penetrationevaluation was completed, the cell was dissembled and examined. Ashutdown behavior was found in the vicinity of the portion through whichthe nail penetrated. From this, it is presumed that the temperature roserapidly to reach beyond the melting point of polyolefin in the vicinityof the nail penetrating portion.

TABLE 4 Porous layer (B) Porous membrane (A) Binder Mem- Number Aver-brane Air of Inorganic age thick- Poros- permea- pores Pore Punctureparticle particle ness ity bility pores/ size strength Type Type sizeExamples um % Sec μm² um gf — — nm Example 16 55 90 130 0.059 470 Alumi-LTX 150 16 num hydroxide oxide Example 16 55 90 130 0.059 470 CalcinedLTX 150 17 kaolin Example 18 61 84 190 0.054 390 Alumi- LTX 150 18 numhydroxide oxide Example 18 61 84 190 0.054 390 Alumi- LTX 60 19 numhydroxide oxide Example 18 61 84 190 0.054 390 Alumi- LTX 460 20 numhydroxide oxide Example 23 55 226 382 0.038 460 Alumi- LTX 150 21 numhydroxide oxide Example 11 66 40 226 0.055 300 Alumi- LTX 150 22 numhydroxide oxide Example 16 80 32 458 0.045 360 Alumi- LTX 150 23 numhydroxide oxide Example 18 61 84 190 0.054 390 Alumi- LTX 150 24 numhydroxide oxide Porous Multilayer porous membrane layer Nail (B) Mem-Increase penetra- Layer brane Air in air Rate tion thick- thick- permea-permea- capa- Evalua- ness Structure ness bility bility bility tionExamples um — um sec sec % ∘/x Example 7 A/B 23 93 3 68 ∘ 16 Example 7A/B 23 92 2 70 ∘ 17 Example 7 A/B 25 87 3 68 ∘ 18 Example 7 A/B 25 90 764 ∘ 19 Example 7 A/B 25 87 3 71 ∘ 20 Example 7 A/B 30 229 4 66 ∘ 21Example 6 A/B 17 43 3 73 ∘ 22 Example 7 A/B 23 35 2 75 ∘ 23 Example 4.4B/A/B 26 92 8 66 ∘ 24

TABLE 5 Porous layer (B) Porous membrane (A) Binder Mem- Number Aver-brane Air of Inorganic age thick- Poros- permea- pores Pore Punctureparticle particle Comparative ness ity bility pores/ size strength TypeType size Examples um % Sec μm² um gf — — nm Comparative 18 61 84 1900.054 390 Alumi- PVA (Soluble) Example 24 num hydroxide oxideComparative 18 61 84 190 0.054 390 Alumi- PVDF- (Soluble) Example 25 numHFP hydroxide oxide Comparative 18 61 84 190 0.054 390 Alumi- LTX 600Example 26 num hydroxide oxide Comparative 18 61 84 190 0.054 390 Alumi-LTX 40 Example 27 num hydroxide oxide Comparative 11 55 33 26 0.09 380Alumi- LTX 150 Example 28 num hydroxide oxide Comparative 16 48 180 1090.055 340 hydroxide LTX 150 Example 29 Alumi- num hydroxide Comparative16 43 230 69 0.06 500 Alumi- LTX 150 Example 30 num hydroxide oxidePorous Multilayer porous membrane Layer Nail (B) Mem- Increase penetra-Layer brane Air in air Rate tion thick- thick- permea- permea- capa-Evalua- Comparative ness Structure ness bility bility bility tionExamples um — um sec sec % ∘/x Comparative 7 A/B 25 109 25 52 x Example24 Comparative 7 A/B 25 130 46 52 x Example 25 Comparative 7 A/B 25 88 467 x Example 26 Comparative 7 A/B 25 94 10 58 x Example 27 Comparative 7A/B 18 54 21 59 x Example 28 Comparative 7 A/B 23 190 10 54 ∘ Example 29Comparative 7 A/B 23 249 19 46 ∘ Example 30

The properties in Examples 16 to 24 are shown in Table 4 and theproperties in Comparative Examples 24 to 30 are shown in Table 5.

It is clear that the multilayer porous membranes in Examples 18 to 20exhibit superior rate capability and safety against nail penetrationcompared to those in Comparative Examples 24 and 25 in which the binderis not a latex. When a latex binder is used, the multilayer porousmembranes in Comparative Example 26 (average particle size: 600 nm) andComparative Example 27 (average particle size: 40 nm) exhibit inferiorsafety against nail penetration. Apparently, the difference in safety isderived from the difference in average particle size.

The multilayer porous membranes in Comparative Example 29 and 30 aresuperior in the nail penetration evaluation while the rate capability issignificantly inferior. Apparently, the porosity contributes to thedifference in the rate capability. In Comparative Example 28, the numberof pores in the porous membrane is small, resulting in inferior ratecapability and safety.

In Examples 16 to 24, rate capability and safety against nailpenetration are satisfied at the same time. The results indicate thatthe multilayer porous membranes in Examples 16 to 24 can be used as aseparator for nonaqueous electrolyte batteries.

EXPLANATIONS OF REFERENCE SIGNS

-   -   3 separator    -   11 positive electrode sheet    -   12 aluminum foil    -   21 negative electrode sheet    -   22 copper foil

This application is based on Japanese Patent Application Nos.2012-074669 and 2012-074689, filed with the Japanese Patent Office onMar. 28, 2012 and Japanese Patent Application Nos. 2012-090420 and2012-090470 filed with the Japanese Patent Office on Apr. 11, 2012, thecontents of which are incorporated herein by reference.

INDUSTRIAL APPLICABILITY

The porous membranes and multilayer porous membranes in Embodiments havesuperior output properties, and have industrial applicabilityparticularly as a separator for lithium ion secondary batteries andlithium ion capacitors in high output applications.

1. A multilayer porous membrane comprising: a porous membrane containinga polyolefin resin as a main component; and a porous layer containing aninorganic filler and a resin binder and laminated on at least onesurface of the porous membrane; wherein the porous membrane has anaverage pore size d=0.035 to 0.060 μm, a tortuosity τ_(a)=1.1 to 1.7,and the number B of pores=100 to 500 pores/μm², which are calculated bya gas-liquid method, and the porous membrane has a membrane thicknessL=5 to 22 μm.
 2. The multilayer porous membrane according to claim 1,wherein the porous membrane has a porosity ε=50 to 90%.
 3. Themultilayer porous membrane according to claim 1, wherein the porousmembrane comprises a resin composition containing polypropylene andpolyolefin other than the polypropylene.
 4. The multilayer porousmembrane according to claim 3, wherein a proportion of polypropylenebased on total polyolefin in the resin composition is 1 to 35% by mass.5. A separator for nonaqueous electrolyte batteries comprising themultilayer porous membrane according to claim
 1. 6. A nonaqueouselectrolyte battery, comprising the separator for nonaqueous electrolytebatteries according to claim 5, a positive electrode, a negativeelectrode, and an electrolytic solution.
 7. A porous membrane comprisinga polyolefin resin as a main component, wherein the porous membrane hasa porosity ε=50 to 90%, and a shrinkage stress at 85° C. of not morethan 2.2 gf.
 8. A multilayer porous membrane comprising: the porousmembrane according to claim 7; and a porous layer containing aninorganic filler and a resin binder and laminated on at least onesurface of the porous membrane.
 9. The multilayer porous membraneaccording to claim 8, wherein the porous layer has a thickness of notless than 3 μm and not more than 50 μm.
 10. A separator for nonaqueouselectrolyte batteries, comprising the porous membrane according to claim7.
 11. A nonaqueous electrolyte battery, comprising the separator fornonaqueous electrolyte batteries according to claim 10, a positiveelectrode, a negative electrode, and an electrolytic solution.
 12. Amultilayer porous membrane comprising: a porous membrane (A) containinga polyolefin resin as a main component; and a porous layer (B)containing an inorganic filler and a resin binder and laminated on atleast one surface of the porous membrane (A); wherein the porousmembrane (A) has a porosity of not less than 50% and not more than 90%,and the number of pores of not less than 100 pores/μm² and not more than500 pores/μm², and the resin binder contained in the porous layer (B) isa resin latex binder having an average particle size of not less than 50nm and not more than 500 nm.
 13. A separator for nonaqueous electrolytebatteries, comprising the multilayer porous membrane according to claim12.
 14. A nonaqueous electrolyte battery, comprising the separator fornonaqueous electrolyte batteries according to claim 13, a positiveelectrode, a negative electrode, and an electrolytic solution.
 15. Aseparator for nonaqueous electrolyte batteries, comprising themultilayer porous member according to claim
 8. 16. A separator fornonaqueous electrolyte batteries, comprising the multilayer porousmember according to claim 9.